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The Carbon Footprint of the Plastic Bottle
Opportunities for Brand Owners and Bottle Manufacturers to save energy and improve their sustainability profile.
The innovation race towards the most sustainable products presents formidable challenges, as many performance indicators are based on fuzzy metrics and uncertain return on investments. That is not the case of the greenhouse gas emissions associated with the energy used in the manufacturing and distribution processes. In fact, carbon footprint can be measured with good precision and has become the common denominator of mainstream sustainability analysis. Plastics are widely recognized as the most energy-efficient packaging material, not only for the advantages offered by their light weight during transportation, but also for offsetting carbon emissions through its recyclability, potential use of renewable materials, advanced design opportunities further reducing weight, and new technologies that require less energy in manufacturing processes.
Eco-profiles and Life Cycle Assessments can help brand owners and packaging converters decide not only which sustainable materials to choose, but also what strategies can best improve sustainability performance, while saving on energy costs during production. Since PET is the fastest growing material used in consumer packaged goods, let’s review the menu of technology options available to designers and to reduce the energy footprint when developing a new product:
1) Materials – The hierarchy is clear: post-consumer recycled materials are better than bio-based (or renewables), which also have a more favorable ecoprofile than fossil raw materials. No matter which of these materials is selected or in which percentage, the final package MUST be designed to be recycled at the end-of-life, ideally back into a new bottle. Despite low oil prices, PET recycling rates keep growing, in part driven by advanced waste management around the world, but also via bottle-to-bottle projects, already reaching 100% PCR content in some markets. According to UK’s Waste & Resources Action Programme (WRAP), recycled PET requires between 10 and 40% of the energy used to produce virgin PET, with additional carbon offset by avoiding waste collection and disposal services (reduction of 1.5-2.5 tons of CO2 per ton recycled). Bio-derived resins, more expensive than virgin, have grown at a whopping 20% annual pace, driving consumers to buy and respect brands that adopt them. Brought to market by Coca-Cola (PlantBottle™, 2008) bio-based PET is moving beyond the initial 30% content, soon to reach 100%; this includes innovative materials like polyethylene furanoate (PEF), a 100% bio-derived resin that generates 50-60% less CO2 emissions than virgin PET.
2) Ecodesign – A good bottle design can help minimize waste by avoiding decisions that could impact the bottle’s recyclability, potentially making it unacceptable to recyclers: label materials, inks, glues, seals, colorants and additives. Some brand owners reinforce the recycling message of their lightweight bottles by asking consumers to twist, flatten, or cap the bottle, remove the label, check if a recycling facility exists, etc. Equipment manufacturers have also contributed with new technologies that deliver thinner walls, special coatings, over stroke bases, lighter necks and shorter closures. In many cases, lightweighting has reached the technical threshold, requiring production and distribution strategies for better stability at the shelf, such as pressurization by Liquid Nitrogen dosing, improved secondary/tertiary packaging/pallet size, and even changes of product claims to accommodate a reduced shelf life. Lightweight bottles clearly contribute to reduce carbon emissions, but they are also responsible for lower yields in the recycling processes. New technologies are being developed to prevent the loss of increasingly thinner wall flakes during the separation of labels and paper residues.
3) Injection molding process – A lot of energy is used to produce a preform prior to blowing a bottle; the granulate PET must be dried, then melted at 270-280°C for injection at high pressure in a mold cavity, which is chilled below 20°C to avoid resin crystallization. But only 5-10% of the total energy used in the process is actually input to the polymer, the other 90-95% being used in mechanical operations. It is estimated that 80% of the energy used in the process goes to the hydraulic pump-motor and 20% is used in ancillary operations, such as dryers, grinders, heaters, chillers and water pumps (source: Plastics Technology magazine’s “Energy Miser” series). All-electric machines and electric-hydraulic hybrids have been replacing conventional technologies, offering higher precision, cleanliness, reduced noise and better energy efficiency (56-78% savings according to a comparative study conducted by the MIT). Many old systems still use fixed speed drives, wasting energy when hydraulic pressure is not required (e.g. downtime during mold chilling). By adopting variable frequency motors and AC drives to power the hydraulic oil pressure pump, operators of injection molding machines can achieve significant energy savings, with an average payback of six to nine months. Additionally, productivity gains can be twice as high as energy cost reductions alone, not to mention that about two-thirds of the wasted energy can be saved through basic no-cost/low-cost management and maintenance improvements.
4) Blow molding process – A re-heat stretch blow molding operation requires even higher input of energy than injection molding. The preforms must be preheated at 90-110°C in infra red ovens, then blown with compressed air at up to 40 bars to fill the bottle mold cavity, which is quickly cooled at less than 20°C. Machine manufacturers are constantly improving the energy profile of their products, including attractive options to retrofit older equipment. More than half of the energy used in blow molding goes into the generation and use of compressed air, which may account for 35-40% of the facility’s total energy bill. The second most important demand comes from preform re-heating, with about a third of the energy consumption. Selecting an efficient air compressor is a good start, but it is even more important to design storage tanks, automation and controls that help save up to 30% on both the supply and demand sides of the compressed air system. Mold cooling and air recovery technologies allow to reduce consumption of compressed air in up to 50%, generally representing a payback time of less than one year for some retrofits. Compressed air leaks are major contributors to energy waste in bottle blowing operations. They can reduce the system’s output in 20-30%, typically worsening the problem when the air lost is compensated with excess capacity. Attention to lamp quality, optimum furnace ventilation and lamp configuration can help establish a stable re-heat process and performance, reducing energy consumption up to 45%. For example, after 5,000 hours, lamps become opaque (internal blackening of the quartz) and require 9% more energy to deliver the same heat. The inefficiency over cost of keeping the lamps in use for another 5,000 hours is on average 50% higher than the cost of a new lamp!
5) Conveyors and Storage – Last but not least, significant opportunities to save energy are associated with the transportation of empty packages inside the plant. Despite the trend to adopt integrated in-line systems to blow and fill bottles, many plants buy preforms and blown bottles, requiring handling, storage and transfer from one equipment to another. The actions of highest potential impact on energy usage by conveyor lines are: a) Selection of state-of-the-art motors, drives and gear reducers - energy savings of up to 8% can easily justify upgrades and replacement of failed standard equipment; b) Specification of motor power to minimize losses due to variable loads – designers can specify two-speed motors and variable frequency drives to enable motors to run near top capacity, considering that variable loads are very common in packaging production and transportation; and c) Monitoring and reporting of energy usage – conveyors must only use the power required for the job and should be automatically paused during periods of lower production, speed must also be increased only when needed to meet throughput demands; energy monitoring and optimization controls usually provided by power monitors upstream of electrical loads (e.g. motors) can help calculate energy savings based on the cost per kilowatt and the hours of motor operation.