Professor Christina Cogdell
15 March, 2018
An Analysis of the Wastes and Emissions Throughout the Lifecycle of Ziploc® Bags
The Ziploc® bags produced by SC Johnson and their off-brand counterparts have become staples of the Western kitchen. The average United States family uses 500 Ziploc bags each year (Bennett). With 325,719,178 people living in the United States as of July 2017, and an average family size of 2.8 people (U.S. Census Data), that means that the United States alone uses an average of 58,164,139 Ziploc bags per year. The rapid consumption of single-use products like the Ziploc bag is a primary contributor to the growing masses of garbage both in designated landfills and as uncollected litter and ocean rubbish. Further, the processes involved in making Ziploc bags contribute to the annual emissions of greenhouse gasses (GHGs), as well as other contaminants into the atmosphere and environment.
Ziploc bags are made of low density and linear low density polyethylene film (LDPE) a common plastic product with many industrial applications. Like most plastics, LDPE is produced from fossil products. LDPE is created through high-pressure polymerization of ethene, an olefin produced from ethane or naphtha, which are derived from natural gas and crude oil respectively. Both natural gas and crude oil are excavated from the earth through drilling processes. There are two primary types of drilling, referred to as conventional and unconventional drilling. Conventional drilling involves building a well, drilling straight down, and retrieving oil using a lift. Unconventional drilling was developed as conventional drilling rigs began to run dry. This method involves hydraulic fracking, shooting high pressure water mixed with toxic chemicals and sand horizontally through an oil well to induce fracturing in the earth that releases previously inaccessible pockets of oil or natural gas. Unconventional drilling has become more common as natural gas and oil deposits have been depleted, and the remaining resources have become more difficult to access through conventional drilling.
Although both drilling methods have the potential to cause environmental harm, unconventional drilling is far more hazardous. “Studies have shown that approximately
3.6-7.9% of the methane from [unconventional] gas production escapes to the atmosphere in venting and leaks over the lifetime of a well…more than twice as great as [emissions] from conventional gas.” (Finkel). These methane emissions contribute heavily to global warming, as methane has 34 times as much global warming potential (GWP) as carbon dioxide. In addition to releasing large amounts of GHGs into the atmosphere, unconventional drilling is also a water contamination hazard. The wastewater produced by fracking is highly toxic and is often not properly cleaned up by the drillers. “Thirty to seventy percent of the fracking fluid will resurface bringing back with it toxic substances” (Finkel), and it is nearly impossible to trace the chemicals that end up in the surrounding environment after hydraulic fracking. These chemicals contaminate ground and surface water (oftentimes the drinking water of residents) and have been shown to have a negative effect on nearby wildlife, causing harm including low fertility and death. (Finkel).
After the natural gas and oil have been extracted from the earth, they must be further refined in order to separate out the ethane and naphtha used to produce ethene. Ethane is a natural gas liquid (NGL) that is separated out from natural gas along with other NGLs during the refinement process due to the high market value of NGLs as isolated materials. There are two primary methods of separating NGLs from natural gas: the absorption method and the cryogenic expansion process. The cryogenic expansion process is used to extract lighter hydrocarbons like ethane. This process requires reducing the temperature of the natural gas to -120 degrees Fahrenheit, then running the gas at a high-pressure through a turbine which will rapidly expand the gas and allow for the recovery of 90-95% of the ethane contained in the gas stream (NaturalGas.org). Achieving such an extreme temperature on a large production scale requires high-energy consumption which produces carbon dioxide emissions from the combustion of crude oil. Naphtha is produced through the distillation of crude oil, a complex heat intensive process utilizing furnaces, boilers, and gas turbines, which is estimated to produce 32.8 tonnes of carbon dioxide per hour of operation (Gadalla). That’s twice as much carbon dioxide as a single resident of California will produce in one year as of 2012 (CCST). Once separated from their sources, ethane and naphtha are steam cracked to produce ethene. There is an estimated .0112 tonnes of carbon dioxide released for every tonne of ethene produced through steam cracking (Ren).
After steam-cracking, ethylene (ethene in its gaseous state) is converted into a solid by heating it at high temperatures in a low-oxygen chamber. This produces LDPE in the form of small white pellets, which are then melted down and hit with high-pressure air in a process called “blowing” that creates LDPE film. This process is typically fueled by crude oil or natural gas, leading to further GHG emissions in the form of burned fuel.
Water-based ink and red and blue dye pigments are used to create a write-on label on the bag and to add color to the zip closure. Ziploc brand bags use Clariant brand dye pigments and a water-based ink produced by Siegwerk. In 2017 Clariant generated .97 million metric tonnes of GHG emissions and 13 million cubic meters of wastewater (Natural Capital - Clariant Integrated Report 2017). As this is the waste generation of an entire company, the wastes associated with the production of dyes for Ziploc bags is likely a fraction of these numbers. Siegwerk does not have any specific numbers associated with their waste production, but do state that they are committed to reducing their production of hazardous waste by 20% by 2020 (Siegwerk's Environmental Performance) (although it is unclear what their starting number is). The hazardous waste to which Siegwek refers is likely large amounts of toxic wastewater, a byproduct of water-based ink production which is difficult to treat, and must be handled carefully to avoid environmental contamination.
The assembly of the Ziploc bag is carried out by SC Johnson at one of their factories. They have many production plants around the globe, including several in the United States, Netherlands, and China. 32.7% of SC Johnson’s energy comes from renewable sources (Sustainability at SC Johnson). They primarily use wind energy, but also use solar energy and biofuel. SC Johnson has also established a zero-waste program and have less than .5 kg of manufacturing waste per 100kg of product shipped (this number pertains to all SC Johnson products, not specifically Ziploc bags), and have reduced global greenhouse gas emissions by 51.7% since 2000. (SC Johnson Sustainability Report). These are fairly significant strides towards sustainability, however, the practices carried out by SC Johnson are only part of the story of a Ziploc bag. Although SC Johnson requires that suppliers of materials follow proper disposal laws for any hazardous materials, they only “encourage” the monitoring of carbon footprint and a commitment to improve environmental performance. These supplier guidelines do not eliminate most of the wastes produced by LDPE, as the hefty wastes produced by processes such as oil drilling and steam cracking are currently completely legal.
The manufacture and sale of Ziploc bags is a multi-continental process. SC Johnson, Clariant, Siegwerk, and the world’s major oil and plastic companies all have locations across multiple countries and continents. It is unlikely that the entire Ziploc production process is conducted in one country. Ziploc bags most likely make at least one trip around the world in their production, and then must be transported to their sale destination from one of SC Johnson’s factories. This transport, whether it is by freight, boat, or aircraft, is powered by fossil fuel or natural gas, and will have carbon emissions associated with the journey. However, given that Ziploc bags are quite small and light, on a large enough transport scale, the presence of Ziploc bags will not contribute heavily to the carbon emissions from the journey.
It is possible to re-use Ziploc bags, provided that one cleans and dries the bag before using it a second time. In fact, there are even some products dedicated to holding Ziploc bags open for effective drying. However, given that this practice is not particularly common, nor specifically endorsed by SC Johnson, Ziplocs are effectively single-use products.
SC Johnson claims that recycling Ziploc bags is as easy as using them. “Yes, it’s true, Ziploc® brand bags are recyclable. Really! Just look for the bin next time you’re at your local participating store. Your used Ziploc® brand bags (clean and dry) go in the same bins as those plastic shopping bags.” (Sustainability & Safety | Ziploc® Brand) While it’s great that the company is promoting the responsible disposal of their product, realistically, a Ziploc bag’s journey from the home to the recycling plant is much more convoluted. LDPE film is categorized as a #4 plastic by United States recycling systems. Recycling #4 plastic is, unfortunately, not as easy as recycling things like cardboard and paper. Not all curbside collection accepts #4 plastic, and of those that do only some accept LDPE film like Ziploc and other plastic bags, while many only accept solid #4 plastics such as milk cartons. Taking Ziploc bags to the grocery store is an option, and many popular chains have collection programs (although not in all locations), however this requires keeping one’s own collection of cleaned and dried bags and remembering to take them to the store. These recycling programs do not offer compensation like plastic bottle and aluminum can recycling programs do, so there is little incentive for the consumer to put in the extra effort to clean, dry, store, and transport a bag rather than just chuck it in their trash bin.
If Ziploc bags do make it to the recycling plant, there are some limitations as to how the LDPE can be recycled. The plastics must be sorted, cleaned, melted down and then re-extruded into plastic pellets. This process is more expensive than making LDPE from new materials, as it includes the extra costs and labor of sorting and cleaning the used material. USDA law prohibits the use of reclaimed LDPE in any packaging that is to come into direct contact with food. As food packaging is a common use for LDPE film, recycled LDPE will often become hard plastic products instead. Common uses for recycled LDPE include garbage cans and trash bags, furniture, and shipping materials. When not recycled, Ziploc bags pose a serious threat to the environment. Like all plastics, Ziplocs can cause harm to aquatic life if they make their way into a freshwater or marine environment. In a landfill environment, Ziploc bags do not decompose, so every bag tossed into a landfill stays there forever.
The use and production of Ziploc bags is both unethical and unsustainable. The plastics, dyes, and inks used in Ziploc bags come from two of the world’s most polluting industries, and their production shows no regard for the need to reduce landfill waste and greenhouse gas emissions. Ziploc bags create environmental damage throughout their entire lifecycle and at the end of their lifecycle, ultimately become waste themselves, as we do not have the infrastructure to effectively recycle the bags on the same scale that they are being consumed. Although
SC Johnson has made efforts to reduce greenhouse gas emissions and waste in their manufacturing process, they are still producing a single-use product with zero biodegradation and limited recycling accessibility.
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Professor Christina Cogdell
15 March 2018
Ziploc Bags: Embodied Energy
Ziploc bags are a fairly common product: after all, the average American family uses 500 Ziploc bags per year (Bennett). Yet, despite their popularity, not many people likely know what goes into producing such a product; after all, the life cycle of any product is not often taken into consideration by the majority of the population. The production of Ziploc bags is an energy intensive process with different possible alternatives that may reduce the energy consumption.
Ziploc bags are composed primarily of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE), plastics derived from ethene (SC Johnson, Lazonby). Ethene itself comes from ethane, which is present in natural gas. As traditional gas supplies decline, hydraulic fracturing, or fracking, of shale gas is becoming a more popular source of natural gas (Clegg). Fracking allows users to obtain gas and oil from impermeable shale rock that was previously unattainable when using conventional drilling. It takes two or more kilometers of drilling vertically to reach the shale, then additional horizontal drilling, up to three kilometers. Slickwater (a mixture of water, sand, and additives) is pumped through the well pipe at high pressure (greater than 600 atmospheres). The horizontal section of the well pipe contains small perforations, creating microfractures in the shale; the proppants, or solid materials (like sand), in the slickwater prop open these fractures, allowing the gas and oil to flow freely from the rock. The gas and oil are captured by a well head once the pressure is released (Lichtarowicz).
Fracking is clearly dependent on water, which begs the question: how much water does this process use? While this depends on the type and location of the well, a single well requires 2.3 to 3.8 million gallons (8.7 to 14.4 million liters) of water. In 2005, the water used for mining accounted for less than 1.0% of the water withdrawn in the US; compared to thermoelectric power generation (49%) and irrigation (31%), this number seems fairly insignificant. While this may be true in the big picture, 2 million gallons (7.6 million liters) per well can often be significant for the area the water is extracted from, as these areas tend to be remote and environmentally sensitive (Chen). In comparison to conventional gas drilling, fracking requires more water. However, using natural gas to generate electricity saves water compared to coal; coal consumes more than twice the amount of water per megawatt hour generated than unconventional gas. In comparison to renewable energies, fracking may use more or less depending on the source: fracked gas uses less than 0.01% of water per unit energy than corn ethanol, but solar and wind use almost no water (Golden).
After obtaining natural gas comes the extraction of natural gas liquids (NGL) like ethane, propane, butanes, and natural gasoline (the main focus here will be ethane). There are two different methods of extracting NGLs: absorption and cryogenic expansion. In the absorption method, the natural gas is passed through an absorption tower, where it comes in contact with “lean” absorption oil. The oil absorbs much of the NGLs, then exits the tower, and the now “rich” absorption oil goes to a lean oil still. Here, the oil-NGL mixture is heated at a point between the boiling point of the NGL and the oil. This yields approximately 75% of the butanes and 85 to 90% of the natural gasoline. The process can be improved to target specific NGLS by refrigerating the lean oil before it enters the absorption tower; this gives a yield of approximately 90% of the propane, 40% of the ethane, and nearly 100% of the natural gasoline. While absorption is good for heavier NGLs, lighter NGLs, like ethane, have a lesser recovery rate. A more effective extraction process is cryogenic expansion, which requires lowering the gas stream to about -120 degrees Fahrenheit. The most effective way of doing this is the turbo expander process, which cools the gas with external refrigerants, followed by an expansion turbine. This expands the gas very rapidly, resulting in a great and rapid temperature drop; hydrocarbons like ethane condense, while methane remains gaseous. This process yields an ethane recovery of 90-95%, and even saves energy costs: the expansion turbine can convert energy from the expansion of the gas into recompressing methane (Maverick Engineering).
Once ethane is obtained, it is converted to ethene by steam cracking, the most energy-consuming process in chemical processing (Ren). Reactant gases (ethane, propane, butane) or liquids (naphtha, gas-oil) are preheated and vaporized; then, in a tubular reactor, they are mixed with steam then heated to 1050-1150 K and converted to low relative molecular mass alkenes. There are a variety of possible feedstocks, and each produces different product yields. Ethene is usually the highest percentage yield product compared to other products, but has the highest yield (78%) from an ethane feedstock (Lichtarowicz). A 1985 energy analysis by the US Department of Energy measured the specific energy consumption of cracking ethane, with a 24% contribution to steam, heating, and losses; this was approximately equal to the other sections (heat of reaction, fractionation/compression, and separation). In the cracking of naphtha, an exergy (usable energy) analysis of an ethylene process from the Tamkang Journal of Science and Engineering attributed de-ethanization (fractionation of ethane) with 23% of exergy loss in compression and separation; this was the second highest loss (to 30% in propylene refrigeration). Perhaps this is due to a greater amount of ethane, but is regardless a significant part of energy usage (Ren).
Finally, ethene is used to produce polyethylene. There are three types of polyethylene: low density (LDPE), linear low density (LLDPE), and high density (HDPE); as stated before, Ziplocs consist of LDPE and LLDPE (Lazonby, SC Johnson). Polyethylene accounts for over 60% of ethene manufactured per year, and is the world’s most important/used plastic. The annual global production of LDPE in 2014 was 18.7 million tonnes, and 24.1 million tonnes of LLDPE. The production of LDPE occurs under extreme pressure (1000 to 3000 atm) at moderate temperatures (420-570 K). Ethene with a purity in excess of 99.9% is compressed, then passed to a reactor with an initiator (eg. small amount of oxygen); molten polyethylene is then removed, extruded, and cut into granules, while unreacted ethene is recycled (Lazonby). A report on the production of 4.48 million tonnes of LDPE (representative of 93.5% of Western Europe production) found the average gross energy to produce one kilogram of LDPE was 77 megajoules (range from 64 to 96 megajoules) (Boustead). Producing LLDPE with high pressure is costly, so improved techniques using catalysts has been developed. Slurry, solution, or gas phase processes can be used with a Ziegler-Natta catalyst, while inorganic catalysts require the use of the gas phase process. The slurry process polymerizes ethene by passing it under pressure into the slurry; granules of Ziegler-Natta catalyst mixed with a liquid hydrocarbon, a diluent, are also in the reaction, which occurs in a loop reactor. Once the product is released, the solvent evaporates, leaving the polymer while leaving the catalyst; to destroy the catalyst’s activity, water vapor reacts with the catalytic sites. The solution process passes ethene and hydrogen under pressure into a Ziegler-Natta catalyst and hydrocarbon solution, obtaining polyethylene in a similar manner to the slurry process. The gas phase process passes ethene and hydrogen over a Phillips catalyst in a fixed bed reactor, resulting the ethene to polymerize into grains that pass out of the reactor once the valve is released. LLDPE is ideal for film products because the material is quite resilient and flexible without needing plasticisers (Lazonby). While I could not find specific numbers for the production of LLDPE, production of LLDPE is greater than that of LDPE, so assumedly LLDPE production uses more energy than that of LDPE.
Ziploc bags also use dye, as notable in their distinctive colored zipper; one side is red and the other is blue, thus when closed the bag shows purple. The dyes, red and blue, used are from the company Clariant (SC Johnson). Clariant’s report on Natural Capital states that their total energy consumption increased 10.0% from 2,950 to 3,245 kWh/t from 2016 to 2017. However, they cite the energy consumption of production as decreasing by 1.8% from 719 to 706 kWh/t. Clariant attributes the increase in total energy consumption to increasing production volume. Clariant has goals to decrease their energy consumption by 30% by 2025 (Clariant). Thankfully, they seem to be mindful of reducing energy consumption, and are perhaps on their way there. Ziploc bags also incorporate water-based ink (SC Johnson). However, I could not find further information on this. Regardless, this should add further energy consumption costs to production.
SC Johnson, the company that makes Ziploc bags, sites their Bay City, Michigan plant as producing Ziploc bags; in 2011, 67% of the plant’s energy was supplied by a nearby wind farm (SC Johnson). In 2017, the company announced it was expanding, also including that the plant produces billions of bags per year, distributing them to seven different countries (The Associated Press). I could not find more information about this, but in terms of transportation, a study for the US Department of Energy provided information of petroleum fuel consumption by US domestic transportation (2005/2007). With the majority of petroleum fuel consumption came from cars and light trucks, 14.2% was combination trucks, 5.1% for single-unit trucks, and 2.1% for freight railroads. Altogether, these do not amount to nearly as much as cars and light trucks (approximately ⅔ of consumption) (Cambridge Systematics). However, the study only takes domestic transportation into account, meaning that total petroleum consumption of transportation must amount to more; in this case, SC Johnson claims they distribute Ziploc bags to seven different countries, thus amounting to further consumption due to transportation, but on an international scale.
Transportation considerations can be similarly applied to waste, as assumedly waste must be transported to various waste management sites. Unfortunately, Ziploc bags are generally single-use products, as reusing them requires cleaning and caring for them (Bennett). I do not believe this process is utilized by many people, as Ziploc bags are widely seen and advertised as a convenient disposable product. Ziploc’s sustainability and safety page claims the majority of plastic bags are made into composite lumber, which is used for various products including fences, benches, and decks; TREX, Hilex, and AERT are sited as “key plastic recycling organizations” (SC Johnson). I continued further investigation of one of these organizations, TREX, which gave me some information on their usage of recycled Ziploc bags. Cleaned plastic waste is ground into granules, which is then combined with sawdust and heated. TREX claims they do not use smokestacks and recycle any waste made back into manufacturing (however, they do not describe how), claiming they produce “virtually zero waste” in manufacturing. In addition to composite lumber products, they also produce LLDPE pellets with surplus material, claiming they are made from 100% recycled materials (Trex). While this all sounds incredibly eco-friendly, claims of “virtually zero waste” and products made from “100% recycled materials” are quite suspicious. However, this report is not on composite lumber or Trex, so no there will be no further research in this paper. Ultimately, it is positive that Ziploc bags are being utilized in another way before they reach the end of their lifecycle.
Ziploc bag production utilizes some controversial and incredibly energy intensive processes (fracking and steam cracking) and are mass produced, mass distributed single-use products. As such, there is constant production, and as the population increases, so does demand, meaning even more production. To reduce energy consumption, people should try to reduce their dependence on Ziploc bags and perhaps consider using reusable storage containers.
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