Sustainability is “Meeting the needs of the present without compromising the needs future” as defined by the United Nation’s Brundtland Commission in 1987. Sustainable design is an important factor in all fields of engineering to insure a safe future with sufficient resources. Unfortunately, due to lack of scientific or engineering knowledge at the time, sustainable design has not always been implemented in engineering. A well-known example is society’s dependance on fossil fuels and their impact on the temperature of the earth. Another issue is the amount of non-degradable solid municipal waste generated. A lot of this non-degradable waste is plastics found in packaging and short life plastic products. The EPA estimates that in 2018 alone, the United States of America generated 35,680,000 tons of plastic waste with 14,530,000 tons (40.7% total) being plastics used in packaging and containers. Plastics can cause potentially devasting long term environmental and economic problems, that is why it is important to reduce the number of plastics discarded. The best way to reduce the number of plastics discarded is to reduce the number of plastics made. This can be done with many emerging alternative options to plastic packaging. Another way to reduce the number of discarded plastics is to reuse plastics. Now, it might be hard to find a new use for a milk bottle if you get one every week. If society took an approach where you brought reusable bottles to the store and filled them up with your desired beverages, this could reduce the number of plastic bottles manufactured just to be discarded soon after. One other way to reduce the number of discarded plastics is to recycle the plastic waste. This article focuses on the engineering, science, and economics behind the recycling and post recycling manufacturing processes of plastics that start out as packaging and are recycled into packaging and non-packaging products. If you are unaware about plastics devasting effects, or want to learn more about the alternatives to plastic packaging read my last article “Plastic Packaging’s Major Flaw; Advancements And Alternatives To Overcoming Plastic Packaging.”
In recent times, many journalists have written about the origin of the plastic recycling industry and how large plastic manufacturers promoted recycling to try to alleviate the public’s fear of buying plastic products. In some cases, plastic is an expectable material especially for long-term applications, but the world would be better off if less single-use packaging plastic was made. These manufacturers promoted recycling so they could sell more plastic packaging without consumer uncertainty. In reality though, 14,530,000 tons of plastic packaging and containers the EPA estimated was disregarded in 2018.The EPA estimates only 1,980,000 tons of it was recycled in 2018 meaning only 13.62 % of plastics disregarded were actually recycled. The idea of breaking down plastic products back to basic plastics and creating a new product is not a bad idea, but a major problem is the low cost of virgin plastics gives little economic incentive for manufacturers to buy recycled plastics. How does the recycling process work and how can it be improved to be more competitive and efficient?
A general overview of the recycling is a product is manufactured and put into use. Then the product is discarded and recycled into raw material. The raw recycled material (post-consumer resin) is then remanufactured into a new product. There are many complications in the recycling system such as sorting, logistics, and the basic chemical make ups of the products. To better understand the details of the recycling process we follow what the process involves for HDPE plastic packaging. HDPE stands for High-Density Polyethylene a common thermoplastic. HDPE is resistant to many solvents and chemical such as oils and greases. It is tough at low temperatures, impact resistant, very stiff, and light weight. HDPE is commonly recycled material and sold as post-consumer resin. In packaging, HDPE is commonly used in bottles and film packaging. It is used in consumer packaging for cosmetics, detergents, and cleaners for its chemical resistance properties. It is also used in plastic bag and milk bottles due to its strength. HDPE certified by the U.S. Department of Transportation and the United Nations for use in hazmat packaging due to its chemical resistance, so it is commonly used in chemical and medical packaging. Now that we know the uses and properties of HDPE, let’s take a deeper look through the life cycle of a HDPE milk bottle from raw materials to the Milk bottle to post resin and the challenges involved in manufacturing a new product with post-consumer resin.
The lifecycle of the milk bottle begins with the making of the bottle, but before the bottle can be made the raw HDPE must be made. HDPE is a petroleum-based plastic and starts out as natural gas. Natural gas is formed in the earth from mostly methane CH4 and hydrocarbon liquids. Natural gas is then drilled out of the ground with hydrolytic fracking. Once the natural gas is transported to a refinery it is converted to ethane by natural gas liquids processing which removes hydrocarbons including ethane by absorbing particles heavier than methane or by using cryogenic expansion and external refrigeration to condense the natural gas liquids process. The ethane is then refined and goes through the cracking process to become ethylene. The cracking process works with the ethane is diluted with steam and being briefly heated to extreme temperatures (between 1292 to 1832 °F or 700 to 1000 °C) in a furnace without oxygen. Due to the intense heating cycle, the gas atoms hit speeds beyond the speed of sound and become olefin gas (a hydrogen and carbon double or triple bonded compound). The olefins are then put through a transfer line exchanger to be cooled down very quickly to produce the greatest yield of ethylene (CH2). This ethylene is now ready for polymerization. Polymerization of HDPE is done one of two ways by using Ziegler-Natta catalyst or the Phillips catalyst both methods of polymerization require low temperatures and low pressures, but we will focus on using the Ziegler-Natta catalysts. Ziegler-Natta catalysts are made of mixtures of transitional metal halides such as titanium (the original Ziegler-Natta catalyst) and non-transitional metal organic derivatives from compounds such as aluminum compounds. Some Ziegler-Natta catalyst process for the polymerization of HDPE also requires a solvent such as propane and water. The ethylene gas, Ziegler-Natta catalysts, and solvent are passed through a reaction machine with a mechanical rotary mixer. The unreacted ethylene is sent back to the main supply of ethylene. The reacted solution is then cleaned with alcohol, the deactivated catalyst is then dumped out through the filter. The solution is then put through a hot water where the solvent returns. The solution is dried and comes out as dried polyethylene. Certain additives are added to the polyethylene depending on the application of the final resin. Now this HDPE polyethylene must become the granule pellets, which are essential to almost all types of plastic manufacturing. The dried polyethylene goes through an extruder to become long strings of HDPE plastics kind of like 3D printer filament. The HDPE strings go through a grinder, which cuts the strings up into the HDPE granules. These granules are the small plastic pellets that are used by nearly almost plastic manufacturer. This virgin HDPE resin is ready to be transported to the milk bottle factory.
This figure from Britannica Encyclopedia shows the Ziegler-Natta polymerization process of ethylene into polyethylene.
Most dairy plants create their own milk bottles for the packaging of their milk, so the HDPE resin is sent over to the dairy factories' milk bottle maker. At the factory, the HDPE resin is put into the extrusion blow molder. There are other ways to create bottles, such as injection blow molding and injection stretch molding, but most milk bottles are made with extrusion blow molding. The extrusion blow molder is used in packaging manufacturing because it is cheaper to operate than injection molders. The extrusion blow molder heats the resin, and it melts. The melted resin is then extruded with an annular die down into the mold around the blow pin into a heated HDPE cylinder. This HDPE cylinder is called the parison. Then a hydraulic pump then blows air through the air pin into the parison, which expands it into the mold shape in our case the shape of the milk bottle. The parison thickness must be controlled very precisely as it can affect the shape and structure of the milk bottle. There are cooling lines in the mold halves to cool the milk bottle after it is formed. The extrusion molders produce the milk bottles with excess plastic called flash around the bottle because the mold pushes the excess plastic to the sides between the crevasses of the mold halves. This excess plastic is cut out the out of the bottle mold with a rotary blade cutter. The regrind can be used again in the extrusion molders, but it will need to be mixed with virgin plastics because each time the regrind is reused it decreases in molecular weight which impacts the strength of the plastic. It is not uncommon for some regrind end up as flash plastic again and be reused 100’s of times! The milk bottles are now ready to be filled with milk. The milk bottles go on a conveyor belt and the milk bottle is filled with milk, labeled, and then the milk is transported. Sometimes milk leaks from the jugs in transport to the store, which is why milk jugs are sometimes sticky. A fun fact about milk jugs are the inverted circles are on them to improve the structure. Now that we know how milk jugs are made, let’s now see how they are recycled.
You now go to the store and buy a milk jug. When you get back you decide to take a cold glass of milk and open up your favorite packaging magazine, PACKCON, to catch up on some packaging trends. After getting lost with time and reading, liking, and sharing dozens of PACKCON articles, you realize you are out of milk and you see the trash can, and your tempted. You know however that you would feel some guilt if you just threw it away instead of recycling it. This feeling of not recycling something is actually a common American feeling and leads to people putting very bazar items in recycling bins. In the sources below, watch Is Recycling Worth It Anymore? The Truth Is Complicated by NPR’s YouTube channel in for more information on recycling guilt. You walk to your sink and wash out the bottle, and then you walk out in the cold or heat to go to the recycling bins in your car port and put your milk bottle in your plastic bin. You then drive your plastics to the recycling bins or your tax dollars go to work for city to pick your plastics. After all the trouble Americans go to and/or their city goes through to recycle their plastics, most are not happy to see that the recycling system has not been incredibly efficient. As the Guardian reported in 2019 shortly after China’s ban on imported plastics:
A Guardian investigation reveals that cities around the country are no longer recycling many types of plastic dropped into recycling bins. Instead, they are being landfilled, burned, or stockpiled. From Los Angeles to Florida to the Arizona desert, officials say, vast quantities of plastic are now no better than garbage. (McCormick, Simmonds, Glenza, Gammon, Americans' plastic recycling is dumped in landfills, investigation shows)
Why has the recycling process not been efficient and what can be done to improve efficiency? But first we must understand how HDPE plastics are recycled and remanufactured into new products.
There are three types of recycling methods mechanical recycling, energy recycling, and chemical recycling. Mechanical recycling does not involve molecular breakdown of materials. It is used to create post-consumer resin for manufacturing products. The two types of mechanical recycling are primary recycling and secondary recycling. Primary recycling (closed loop recycling) is when uncontaminated products are broken down and recreated into the exact same product. Secondary recycling is when products that need to be sterilized are manufactured into other resin to be used in other products. Energy recycling is where plastics are incinerated to create energy. Chemical recycling (sometimes referred to as Tertiary recycling) is the process of breaking plastics down chemically back into petroleum products. We will focus on secondary mechanical recycling. After the plastic recyclables arrive at the recycling center, they are then sorted by color (in some cases) and type of plastic as most Americans send multiple types of plastics in the same bins to the recycling center. There are many methods for plastic sorting that may involve float tanks, rotary separators, humans, and even robotics and computer vision. Rotary separators spin the plastics around and use air pressures to lift low density plastics out of the rotary onto a separate conveyor belt; these plastics tend to be polyethylene and polypropylene. The remaining high-density plastics can be sorted with near infrared light systems that use the plastic’s reflections to identify plastics, and more recently cameras and computer vision are used to identify and sort plastics. Once plastics are identified, robotics of some type are needed to carry out the sorting of the plastics. Some recycling plants use float tanks to separate plastics based on destiny, and the viscosity of the water can be controlled by adding chemicals depending on the types of materials needed to be sorted. Some recycling plants still employ humans to sort the plastics. The HDPE only plastics need to be sorted again as there are different grades of HDPE, which have different thicknesses and abilities. Sorting is still a major issue in the HDPE recycling process as it is not uncommon for polyethylene plastics to be mixed with the HDPE. If the polyethylene (PET) amount is over 10%, this can cause major issues when the post-consumer resin is used in the manufacturing of a new products. The HDPE recyclables are then ground to flakes using a single shaft shredding system. The single shaft shredder requires the HDPE recyclables to be fed through the hopper on the single shaft shredder and the recyclables then goes through solid steel rotor with cutters and stationary cutters in around the rotor. The HDPE is now HDPE flakes that are ready to be decontaminated. The plastics then go through an intensive cleaning phase where labels and past debris are removed. The cleaning systems use either hot or cold-water solutions with certain detergents depending on the conditions. Some recycling plants deploy mechanical cleaning, which utilizes vibration plates, compressed air, and centrifuges. It is important in this step to get rid of the remnants of the milk bottle’s label. Mechanical cleaning methods are also used in drying off the plastics. After the HDPE flakes are cleaned and dried, the HDPE flakes are then melted and go through an extruder to again become the HDPE strings. These HDPE strings are again put through the grinder to become the HDPE granules. These granules are now the HDPE post-consumer resin, and this resin can be used to manufacture new HDPE products. We know how milk bottles are manufactured with virgin resins, but what challenges must be overcome to effectively manufacture packaging and non-packaging items with post-consumer resins? And what sustainability benefits come with post-consumer resins versus virgin resins?
An overview of the recycling process from Technical Discussion over Three Means of Waste Management
In the coming years packaging engineers must learn how to engineer packaging that contains post-consumer resin for the good of the planet and now many governments are setting deadlines. By 2025, the European Union is requiring all plastic packaging to contain at least 25% recycled content, and by 2030, the European Union is requiring all plastic packaging to contain at least 30% recycled content. By 2030, Canada is aiming that all plastic products be contain at least 50% post-consumer resin, and California is requiring all plastic bottles contain 15% post-consumer resin by 2022 (next year). What are the challenges and differences with manufacturing plastic packaging with post-consumer resin? Currently, one major challenge lies in the economics of post-consumer resin. There is a lot of demand for post-consumer resin, but not enough supply to allow all the plastic product plants to have post-consumer resin. This is due to a shortage of recycling being done globally. An extra challenge for food and drug packaging and product makers is sourcing FDA approved post-consumer resin. There are currently many companies trying to get FDA approval for their post-consumer resins, but there are a few companies that have been approved. The supply issues and post-consumer resin manufacturers drive to create higher quality higher priced products is resulting in virgin resins being cheaper to source in many cases. The pricing of post-consumer resin can decrease with more FDA certified manufacturers and more access to plastics for recycling. Furthermore, prices of post-consumer resins are predicted to stay about the same, but petroleum’s prices are predicted to increase, which will impact the cost of virgin plastics. Another challenge is many post-consumer resins turn out in a gray color. The color of the plastics is not a structural issue, but for packaging and product designers, who want the packaging or product to be a particular color, this can be harder to work with. There can also be small defects in the final product such as miscolored flakes and becks if the plastics where not properly sorted or decontaminated. Remember how excess flash plastic in the milk bottle manufacturing had to be mixed with virgin plastics because of the decrease in molecular weight, well this can be a challenge with manufacturing with post-consumers resins. Post-consumer resins have less molecular weight due to thermal and hydrolytic degradation in the recycling process. In the recycling process additives and fillers are added to try to minimize the effects of lower molecular weights, but post-consumer resins are still not as good for manufacturing as virgin resins. Also, the original additives put into the HDPE plastic are hard to separate from the plastics in the recycling process, and these additives lower strength in the final products. UC Berkley has been conducting research in using strong acids to remove additives, and they have had success maintaining a similar quality of manufacturing to virgin resins. These challenges are fixable with continued research into fixing these issues will make post-consumer resins are more competitive option for manufacturers.
Recycling processes can be improved to minimize the disadvantages of post-consumer resins. As alluded to in the previous paragraph, more recyclers would mean more competition and more recyclables could be processed, which could lead to higher supplies and lower prices. Also, if Berkley’s methods for removing additives could be applied economically to large scale operations would help increase the strength of the recycled plastics. Probably the biggest area in need of change in the recycling process is the sorting. Sorting issues can cause major problems with strength and appearance. There are 7 different plastic recycling codes, but PET and HDPE make up 97.1% of the plastic packaging market. With these numbers, packaging engineers should design new consumer packaging to only use HDPE or PET when possible. For example, some yogurt cups are made from PP plastic. HDPE plastic is used to hold milk another dairy product, so could HDPE yogurt cups be engineered? Packaging engineers should also focus on creating packaging with only a single type of material, so that it can be recycled easily without having to be broken down. Because packaging is recycled daily and HDPE and PET make up the majority of packaging, governments or recycling companies should require individuals to sort their plastic by type or at least by HDPE, PET, and other, but a concern may be that people will not recycle as much as it takes more effort. Even with these measures, some consumers may not properly identify their packaging and put it in the wrong bin. This is where advancements in computer vision and robotics can assist, and these methods are being pioneered by Amp robotics. Outside of computer vision, data science in general can be used to improve recycling. Collecting consistent data on the types of plastics received, such as additives in them, color, type, density, application before recycling, quality after recycling, etc, can be used to improve recycling and understand how to optimize the recycling process in terms of sorting and quality. And for the defects or different plastic colors, packaging designers must deal with can be used to market the brand as a sustainable brand, which resonates with younger consumers. There are many ways to improve the recycling process, but how optimal is it currently?
In terms of saving the environment, our current recycling process is working. Post-consumer HDPE requires 88% less energy to manufacture and results in a 71% decrease in overall emissions. This does make sense if you recall how that virgin HDPE requires fracking, refinement, cracking, being polymerized, and being granulated, where-as in mechanical recycling, post-consumer resin requires sorting, grinding, cleaning, and being granulated. In terms of water consumption during manufacturing, post-consumer HDPE requires 29% less water than virgin HDPE. At its fundamentals, recycling allows us to keep the plastics in use and out of landfills while simultaneously preserving our natural resources. This goes back the United Nation’s Brundtland Commission definition of sustainability “Meeting the needs of the present without compromising the needs future,” and with intelligent engineering, we can overcome the additional costs and challenges of using recycled plastics to allow the earth to continue to prosper!
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