The many faces of bioplastics

The many faces of bioplastics, The world of eco-friendly­­, recyclable­ plastics is more complex­ than it appears­ on its slick, shiny surface­.
July/August 2013
The world of eco-friendly­­, recyclable­ plastics is more complex­ than it appears­ on its slick, shiny surface­.

Plastics get a bad rap. While they improve our lives in countless ways, from food safety to medical care, their inertness and disinclination to break down — the very properties that make them so useful in most applications — raise the spectre of pollution. Spurred on by images of the vast collection of marine debris known as the Pacific trash vortex, plastics manufacturers are scrambling to market eco-friendly versions of their products. As a result, the term ‘bioplastic’ is increasingly found on everything from food packaging to water bottles. But should these ostensibly greener bottles go in the compost, the recycling bin, or the regular trash? It turns out that bioplastic is a term that means different things to different people, and the correct answer largely depends upon your point of view.

Plastics recycling 101

To Mohammad Rahbari, plastic is far from a dirty word. “By definition, thermoplastics can be continuously re-melted and reformed to have a multiple-use history,” says Rahbari, who works as Director of Technology Development at Sarnia, Ontario-based Entropex, one of Canada’s biggest plastics recycling­ operators. By ‘thermoplastics,’ Rahbari means the tried-and-true industrial resins labelled with the ubiquitous recycling symbols 1 through 7. Until recently, technological and economic limitations meant that recyclers like Entropex could only make a living from recycling two of the seven types: high-density polyethylene (HDPE, or type 2) and polyethylene terephthalate (PET, or type 1) from which most clear plastic water bottles are made. Of course, that didn’t stop people committing ‘bin sins’ — throwing other types of plastic into their blue boxes. “Thirty per cent of the stream consisted of plastics that could not be processed by conventional recyclers,” says Rahbari.

In 2010, Entropex underwent a major upgrade at their 180,000 sq. foot plastic recovery facility. “Essentially we’ve built our business on bin sins,” says Keith Bechard, president of Entropex. “We want you to throw in all the different types of plastic you have, because we have the capability of sorting and processing them into highly pure resins.” But some types cause more trouble than others. For example, polyvinyl chloride (PVC, or type 3) can degrade if re-melted and because it contains chlorine, it can form hydrochloric acid or other hazardous chemicals. These features limit its potential for re-use. “Entropex lacks a reliable domestic market for its PVC; it currently is sent to landfill,” says Bechard.

So where do bioplastics fit into all this? “That term means a bunch of different things,” says Bechard. “We love bioplastics, provided that they’re compatible with the existing stream. If they’re not, then they’re a contaminant, and that’s where we have issues.”

Compatibility versus compostability

“Very simply, we make plastics from plants.” That’s Toby Reid, president of Solegear Bioplastics, a Vancouver-based company that provides bioplastic solutions for packaging, electronic casings, children’s toys and more. According to Reid, the ‘bio’ in bioplastics refers to the source of the raw materials. Unlike traditional plastics that come from petroleum, bioplastics come from biomass. Technically, that definition includes cellulose (paper) and natural rubber. But when most people think of bioplastics, they’re thinking of polylactide (PLA). Lactide is made from lactic acid, which in turn is a natural product of the bacterial fermentation of sugar or starch. In Reid’s case, the raw material is #2 yellow dent field corn, a non-food variety. The industrial-scale fermentation of corn to lactic acid and its subsequent polymerization to PLA is carried out by NatureWorks, co-owned by Cargill and PTT Global Chemical, and the world’s biggest producer of the material.

NatureWorks sells its raw PLA to companies, including Solegear, which mix in various chemical additives to alter its properties. “While other players in the industry tend to use petroleum-based additives, we’re focused on using as much bio-based content as we possibly can, ensuring that our final product is compostable,” says Reid. Solegear’s formulations comply with the American Society for Testing and Materials (ASTM) standard known as D6400, meaning they will biodegrade under the aerobic conditions found in municipal and industrial compost facilities. “We conducted a test at the Vancouver composting facility and our material degraded in eight weeks,” says Reid. For municipalities that don’t have compositing facilities, Reid says that his PLA-based formulations can be recycled, that is, melted down and re-formed into new products, just like any other thermoplastic.

Solegear's bioplastic products, such as this air freshener tray, are based on bio-sourced and biodegradable polylactide (PLA) with nontoxic additives mixed in to optimize the properties of the material. Unlike many other formulators, Solegear's formulations use no toxic plasticizers to improve performance and are biodegradable in municipal compost facilities. Credit: Kent Kallberg

But recyclers like those at Entropex are skeptical that PLA-based products will actually find their way into municipal composting facilities. “For 30 years, we’ve trained consumers such that if they see plastic, they put it in those little blue bins,” says Rahbari. In Rahbari’s world of recycling labels, PLA is listed as type 7, a catch-all category for anything that doesn’t fit into types 1 through 6. Entropex has the technology to sort out type 7 from the other streams, but because its composition is unknown, it can’t be recycled. “The only option for type 7 is to go to a landfill, where it gets entombed,” says Bechard. The oxygen levels, heat and other conditions found in a landfill are very different from those found in an industrial composting facility, meaning that PLA may not degrade there even if it is degradable. Moreover, even if recyclers created a new category — say type 8 — for PLA-based products, Bechard says the amount of it currently on the market is so low that the cost of recovering it exceeds what he could sell it for. “Technically it’s recyclable, but practically it’s not.”

Rahbari and Bechard say that the ideal solution is to create plant-derived plastics that are chemically indistinguishable from their oil-based cousins. This approach is exemplified by the PlantBottle, a type 1 PET product developed by Coca-Cola. “The PET polymer at least 20 per cent monoethylene glycol; the rest is terephthalic acid,” says Rahbari. “What they’ve got is monoethylene glycol that comes from sugarcane ethanol, from Brazil.” This means that while the PlantBottle is at least 20 per cent bioplastic, it integrates seamlessly into existing PET streams. Of course, this assumes that people don’t mistake the PlantBottle symbol for one that indicates a biodegradable plastic like PLA, and put it in a municipal compost facility.

With a PlantBottle (20 per cent bioplastic) that is recyclable but not compostable, and a PLA bottle (100 per cent bioplastic) that is compostable but not easily recyclable (at least with the infrastructure that’s currently in place) confusion is rampant. Reid says one of his biggest challenges is consumer education. “Many developing industries require communications that take very complex concepts and boil them down into understandable sound bites,” he says. And consumer confusion isn’t the only issue facing bioplastics.

Polymer Performance

In early 2010, Frito Lay Canada announced the “world’s first 100 per cent compostable chip bag.” The PLA-based product, used to contain the popular SunChips brand corn chips, met the ASTM D6400 standard, degrading into compost-ready soil within 14 weeks. But chip fans noticed another property: the PLA bag made a loud, annoying sound when crinkled. Frito Lay unsuccessfully tried to use the property to its advantage, calling it the “sound of green.” Still, unhappy customers caused the company to return to a non-biodegradable bag.

The noisiness of PLA is caused by its molecular structure, which is more rigid than many other plastics. It may be a drawback for potato chip bags, but the rigidity helps products stand up to extreme heat or long-term storage. The key is to be able to adjust and control properties like rigidity in order to optimize the polymer for various applications.

Parisa Mehrkhodavandi, a professor of chemistry at the University of British Columbia, is doing just that, “looking to improve PLA’s properties at a molecular level.” Lactide, the monomer from which PLA is made, is a chiral molecule, meaning it has two possible orientations: right-handed D-lactide and left-handed L-lactide. Traditional PLA is a random, uncontrolled mix of the two, but Mehrkhodavandi’s goal is to create new chains with carefully controlled compositions, for example, a repeating pattern of 1,000 units of L-lactide followed by 1,000 units of D-lactide, known as stereoblock copolymers.  By controlling the length of each polymer block, she can tailor the rigidity, heat tolerance and other properties. “What you need is a catalyst that can distinguish between L and D and preferentially polymerize one over the other,” says Mehrkhodavandi. Aluminum-based catalysts capable of doing this have been reported in the past, but they’re very slow, often taking more than a week to complete the reaction.

The lactide monomer has both left-handed (green) and right-handed (blue) enantiomers. Stereoblock polylactide (PLA) made by ring-opening polymerization has a set number of one enantiomer (n) followed by the other (m.) Selective catalysts developed at the University of British Columbia can produce such stereoblock polymers, which have improved properties over amorphous PLA.

Last year, Mehrkhodavandi’s team reported a new catalyst system with two atoms of the metal indium at its core. The complex selects L-lactide over D-lactide about 80 per cent of the time and takes minutes as opposed to days to carry out the reaction. This allowed the team to add L-lactide, D-lactide or random mixtures of the two in careful sequences. In May, the group published a paper in Macromolecules that showed how a similar indium-based catalyst could be used to create triblock copolymers with higher tensile strength than amorphous PLA, meaning that while they might not be ideal for potato chip bags, they could make for strong rigid packaging.

The team went further and made a polymer that contained blocks made of a completely different polymer: poly (3-hydroxybutyrate) or PHB. PHB is produced by some bacteria as a way of storing carbon for lean times, but like PLA it can be used as a bio-sourced, biodegradable thermoplastic.  Mehrkhodavandi’s indium catalyst is able to react with β-butyrolactone, the monomer from which PHB is made, and add it on to a PLA chain or vice versa, allowing for the creation of PLA-PHB-PLA triblock copolymers.  While this material turned out to have lower tensile strength than PLA-only chains, it was able to stretch by up to 20 per cent before breaking, compared to only five per cent for PLA. With further improvements, such formulations could be used for applications where stretchiness is important, such as children’s toys.

The future

While researchers like Mehrkhodavandi are working hard to improve the qualities of bioplastics like PLA in order to get them into more markets, their current piece of the pie remains small. But bioplastic formulators like Solegear are undeterred. “Hybrid cars were invented in the 1920s, but it’s only recently that they’ve found a viable market opportunity,” says Reid. “What we're seeing in the PLA ecosystem is the same dynamic.” Reid says he’s not out to completely displace traditional petroleum thermoplastics, with their well-established infrastructure. Instead, he sees his product evolving alongside what’s already out there. “There’s no such thing as bad plastics, only bad applications of plastics,” says Reid. “Our name refers to us being one ‘sole gear’ in the bioeconomy. We’re not saying that we’ve got all the answers, but we certainly believe we’ve got a role to play, and we think that our country does too. Canadians should be very excited about that.”


Sidebar: oxo-degradable plastics

There is a third option to traditional petroleum-based plastics (recyclable but not degradable) and plant-based PLA (degradable, but not easily recyclable). Companies like Vancouver’s EPI Environmental Technologies have created special polymer additives  that include a metal salt like cobalt stearate or manganese stearate. In the presence of oxygen, these additives act as catalysts to accelerate the natural breakdown of petroleum-based polymers into microscopic pieces; going from decades to one to three years.

But additives of any kind can be a nuisance for recyclers, as they interfere with the purity of the recycled product. Steve Alexander, director of the Association of Postconsumer Plastic Recyclers, stated in a media release that there is “no credible supporting data” supporting the claim that additives wouldn’t impact the recyclability of bottles. In response, EPI pointed to a 2007 study by the Centre de recherche industrielle du Québec which showed that under accelerated aging conditions (high UV, heat, humidity) plastic containing EPI’s additive degraded at the same rate as normal recycled plastic, lasting for about seven days. However, bags containing other oxo-degradable additive formulations crumbled after only four days.