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The not-so-humble plastic bottle.

Plastic bottles are not known for being at the forefront of technological development, but they are when it comes to sustainability, and this can occasionally lead them to transform entire industries. In 2009, the Coca-Cola Company launched the PlantBottle, a polyethylene terephthalate (PET) bottle made partly from plant material. Since then, the company has distributed over 10 billion of these bottles across 24 countries and licensed the PlantBottle to several other companies, helping to make bio-based PET the world's most manufactured bioplastic.

According to the trade association European Bioplastics, bio-based PET currently accounts for more than 40% of global bioplastics production capacity, at around 450,000 tonnes. What is more, bio-based PET is likely to drive much of the growth in global bioplastics production over the next few years. European Bioplastics predicts that bio-based PET production capacity will increase to 4.6 million tonnes by 2016, by which time it will account for 80% of global bioplastics production capacity. In just three years, bio-based PET has come from nowhere to achieve a commanding dominance of the global bioplastic market, and all because of plastic bottles.

It's not just bioplastics, either; plastic bottles are also leading the charge for other forms of sustainability, including lightweighting and recycling. According to the US National Association for PET Container Resources (NAPCOR), the weight of a 1L bottle of detergent has fallen by 64% since the 1970s, while the weight of a 500ml bottle of water has fallen by more than half over the past 20 years, from almost 25g in 1990 to around 10g in 2010. Lighter bottles not only require less material to produce them, but also require less energy to transport them, greatly reducing greenhouse gas emissions.

Plastic bottles are also some of the most widely recycled plastic materials. In the US, almost 30% of plastic bottles were recycled in 2011, while across Europe the recycling rate for PET bottles was 51%, although that figure varies widely between countries. A survey of PET bottle recycling rates in 31 European countries conducted for the European trade association Petcore (PET Containers Recycling Europe) found that a third of countries achieved recycling rates of more than 70%.

In both Europe and the US, the recycling rate for plastic bottles is much higher than for plastics in general. According to the US Environmental Protection Agency, only around 8% of plastics were recycled in the US in 2010, while the trade association Plastics Europe reports that around 24% of plastics were recycled in Europe in 2011.

So the humble plastic bottle is at the cutting edge of three separate sustainability initiatives. The main reason for this is that although the plastic bottle may be humble, it is also ubiquitous.

Huge Volume Production

Plastic bottles are manufactured in such huge volumes that reducing the weight of a single plastic bottle even slightly produces huge overall savings in materials and energy. Meanwhile, it's not much of an exaggeration to say that the whole global recycling industry is built on plastic bottles. This is both because plastic bottles are a large source of waste plastic and because the vast majority of them are made from just two different types of plastic, both of which can be readily recycled. And making a plastic bottle from plant derived material transformed the whole bioplastic sector in just three years.

In 2011, around 4 million tonnes of plastic bottles were produced in the US and around 3.5 million tonnes were produced in Europe. The vast majority of these bottles were made from PET, accounting for 60-65% of bottle production in both the US and Europe. The second largest plastic type used in bottle manufacture is high density polyethylene (HDPE), accounting for around 36% of US production in 2011. Holding up the rear are low density polyethylene (LDPE), polyvinyl chloride (PVC) and polypropylene (PP), all of which account for less than 3% of US production.

As PET is lightweight, comparatively cheap and poses a low risk of leaching, it is mainly used to produce soft drink bottles. HDPE also poses a low risk of leaching, but tends to be more robust and chemically resistant than PET, so it is used to produce bottles for household cleaners, detergent and shampoo, as well as for milk and fruit juice. Because LDPE is more flexible than HDPE, it is used to produce squeezable bottles for products such as shampoo and hand cream. Like HDPE, PVC is tough and chemically resistant and is used to produce bottles for liquid soap, shampoo and detergent. PP has a high melting point, so it is often used to produce bottles that are initially filled with hot liquids, such as syrup bottles.

Losing Weight

As we have already seen, the weight of both PET and bottles has fallen sharply over the past few decades, with the weight of HDPE shampoo bottles falling by 64% since the 1970s and the weight of PET water bottles more than halving since 1990. Still, plastic bottle manufacturers are continuing to look for ways to reduce the weight of their bottles, although any reductions tend to be fairly modest now.

This is because plastic bottles still need to be sturdy enough, with thick enough walls, to hold their liquid contents. For PET soft drink bottles, this means ensuring they don't buckle or distort under the pressures generated by carbonated beverages. For HDPE bottles, it means ensuring that the chemical products can't leak out and that any gases generated don't cause the bottles to distort. The practicalities of the bottle manufacturing process, which is usually based on blow molding, in which a small plastic preform is expanded into a full-size bottle by pumping air into it, can also place limits on material reduction.

Although only modest weight reductions are now usually achieved, they can still be worth it, both financially and environmentally. A recent project conducted by UK PET bottle manufacturer Esterform Packaging for the UK Waste & Resources Action Programme (WRAP) showed what is still possible.

Design Considerations

By tweaking the design of the preforms, Esterform Packaging was able to reduce the weight of a 2L bottle of soft drink from 42g to 40g and reduce the weight of 500ml water bottles from 25g to 20g. One of the main problems the company had to overcome in achieving these weight reductions was ensuring that the neck opening didn't become larger than the diameter of the preform body, which can cause problems during blow molding. This difficulty comes about because only the preform body is expanded during blow molding, which means that the neck opening in the preform has to be the same size as in the finished bottle.

To get around this problem, Esterform designed preforms that were shorter and wider than normal. Although the resultant weight savings were only fairly minor, amounting to under 5% for the soft drink bottle but a more substantial 20% for the water bottle, they can still have an impressive overall impact. WRAP calculates that if these new bottle designs were adopted throughout the UK soft drinks industry, they would save 3,400 tonnes of PET material at a cost of 2.7 million (pounds sterling), as well as 2811 million kWh of energy and 276 tonnes of carbon emissions.

Other companies are also finding ways to achieve weight reductions. Husky, a Canadian manufacturer of injection moulding systems and PET preform molds, offers several solutions for lightweight plastic bottle preforms. These include the EcoBase PET preform, which contains a modified base that makes it up to 2.5% lighter than other plastic bottle preforms, and preforms with very thin side walls, at just 1.5mm thick.

These kinds of weight savings require a fine level of control over the preform manufacturing process. "In order to have very, very lightweight parts, your (manufacturing) system not only needs to be high performance, but it also has to have very tight process control," explains Rob Lovell, business manager for PET systems at Husky. "It needs to operate within a very tight band of process parameters or you're not able to make those very light preforms."

Another example is provided by US company Plastipak Packaging with its ThermoShape system, which won a DuPont Award for Packaging Innovation in 2011. As it's name suggests, ThermoShape is designed for holding hot fill beverages. In this case, drink products such as fruit juices are poured into bottles at high temperatures to ensure that both they and the inside walls of the bottle are free of living micro-organisms, allowing them to stay fresh longer.

The problem with this technique is that the liquids shrink as they cool, creating an internal vacuum that can damage the bottles. As a consequence, hot fill needs to be done in bottles made from PP or PET bottles with thick walls. The ThermoShape system, on the other hand, uses a PET bottle with thin walls that deforms as the liquid cools, but then employs a machine to stretch the bottle back to its original shape. In this way, PET bottles containing up to 20% less material can be used with hot fill beverages.

Recycling Bottles

Although all these efforts to reduce the weight of plastic bottles can save a lot of material, with all the associated environmental benefits, they do cause a problem for another important sustainability initiative: recycling. This is because the amount of revenue that recycling companies make obviously depends on the amount of material that they recycle. However, lighter bottles mean less material for recycling.

In its 2011 report on US PET recycling activity, NAPCOR highlights lightweighting of plastic bottles as one of the main issues affecting the recycling industry. Lighter bottles mean that companies at all stages of the recycling process have to handle more bottles to obtain the same weight of material, thereby increasing costs and creating logistical issues.

Despite this, however, the recycling rate for plastic bottles is continuing to increase. "US reclaimers purchased a record 916 million pounds of post-consumer bottles in 2011, and 1.04 billion pounds of recycled PET went back into new product applications;' says Tom Busard, NAPCOR chairman and vice president of global procurement and material systems for Plastipak Packaging. "These increases happened despite the effects of PET bottle lightweighting, which really played out this year to a greater extent than we've seen to date. The incidence of lighter bottles in the (recycling) stream reflects manufacturers' commitment to improving PET's environmental footprint, but means additional handling for PET recyclers in the short term in order to produce the same weights."

According to a report jointly published by the American Chemistry Council and the US Association of Postconsumer Plastic Recyclers, the weight of all plastic bottles recycled in the US in 2011 rose by 1.7% to 2,624 million pounds, representing a recycling rate of 29%. Over the past five years, the US recycling rate has increased by an average of 3.4% a year, although lightweighting means that the amount of material being recycled does not always follow suit. For example, while the recycling rate for HDPE in the US stayed the same in 2010 and 2011 at 30%, the actual weight of recycled HDPE bottles fell by 10 million pounds in 2011 to 974 million pounds, simply because of lighter bottles.

Reflecting their dominance in the plastic bottle market, PET and HDPE bottles also dominate plastic bottle recycling, accounting for 98% of recycled bottles in the US. In addition, the recycling rate for PET and HDPE bottles is much higher than for other types of plastic bottles: the US recycling rate for PP bottles is 21%, while for PVC and LDPE bottles it is just 2%. This is purely because the infrastructure is not yet in place to recycle these bottles.

Once recycled, PET bottles can be turned into clothing, furniture and carpets, while HDPE bottles can be turned into pens, floor tiles, drainage pipe and, rather aptly, recycling containers. Ideally, though, old plastic bottles should be recycled into new plastic bottles, thereby creating a closed manufacturing loop. The problem with doing this is that the physical characteristics of recycled plastic resin tend to be more variable than virgin resin, because the recycled resin comes from various sources, which can be an issue when manufacturing preforms.

Increasingly, however, plastic bottle manufacturers are developing ways to deal with this variability. For example, Husky announced in 2010 that it had optimized one of its preform manufacturing systems, known as the HyPET Recycled Flake (RF) system, to produce preforms with up to 100% recycled material. Previously, the system had only been able to utilize up to 50% recycled material.

"This is the only system in the world that's been specifically designed to handle recycled flake efficiently," claims Lovell. Husky has already sold these systems to companies in Europe, South America and Southeast Asia, where one company is even creating new food-grade plastic water bottles using recycled flake produced from bottles dug up from a landfill.

As it's made from exactly the same PET, Coca-Cola's PlantBottie can also be recycled. PET is made from 30% ethylene glycol and 70% terephthalic acid. Ordinarily, both the ethylene glycol and terephthalic acid are derived from petroleum, but in PlantBottle, the ethylene glycol is derived from sugar cane via ethanol, meaning that the bottle consists of around 30% plant material.

By 2020, Coca-Cola intends to use PlantBottle for all its soft drink bottles. To this end, in September 2012, it announced a partnership with the Indian plastic producer JBF Industries to build the world's largest facility for producing bio-based ethylene glycol in Sao Paulo, Brazil. It has also licensed the PlantBottle technology to several other companies, including Heinz, which is using PlantBottle for its tomato ketchup.

Ultimately, however, Coca-Cola wants to produce PET entirely from plant material, which means solving the much greater challenge of finding a way to derive terephthalic acid from plant material. In December 2011, Coca-Cola announced multi-million dollar partnership agreements with three biotechnology companies--Virent and Gevo in the US and Avantium in the Netherlands--to do just that. Each company has come up with a potential solution to the terephthalic acid problem, but all the solutions are very different.

Virent and Gevo have both found ways to convert plant material into paraxylene, which can then be converted into terephthalic acid via a simple oxidation reaction. Virent has developed a catalytic process for transforming plant material into a range of useful hydrocarbon molecules, including paraxylene. Gevo has developed a genetically-modified version of yeast that can produce isobutanol, which can easily be converted into paraxylene.

In contrast, Avantium has developed an efficient catalytic process for transforming plant material into an organic molecule known as furanoate, which can replace terephthalic acid in PET to produce a plastic with very similar properties called polyethylene furanoate. Coca-Cola will now invest in the commercialisation of these three processes, with the aim of producing the next generation of PlantBottle made entirely from plant material. To spur these efforts, in June 2012, it joined forces with Heinz, Procter & Gamble, Nike and Ford to form the Plant PET Technology Collaborative.

Coca-Cola's great rival, PepsiCo, has also announced the development of a PET bottle made entirely from plant material, although it has not revealed any details of the process. Furthermore, with the Brazilian plastics producer Braskem starting up the world's largest facility for producing polyethylene from sugarcane-derived ethanol in 2010, bio-based HDPE bottles should not be too far behind.

So while marvelling at the advanced technology that went into making your new tablet computer or smartphone, don't forget about the equally advanced technology that has gone into making the latest versions of the humble plastic bottle.

Jon Evans is a freelance writer based in the United Kingdom.
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Author:Evans, Jon
Publication:Plastics Engineering
Article Type:Cover story
Date:Jan 1, 2013
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