Throughout this article it is essential to understand that “plastic” is not a simple material which is not assembled or designed. Some very sophisticated assembly is needed to push molecules into alignment and connection and that assembly takes lots of money, time and effort, likely more than that needed to assemble fibers into clothing or transistors into computers. All dedicated molecules represent high levels of hard won assembly which must be conserved through reuse. Burning and even composting are terribly destructive ideas which discard all of the expensive assembly that created the molecules.
In a recent article in Science (August 2009) there was a long discussion of new ways to polymerize olefins (ethylene, propylene etc.) using different catalysts that join the polymers in all different ways to achieve many different properties.
In that article, it was taken for granted that “…Finding the balance between performance and price is critical to commercial success, as the customer will always adopt the cheapest solution that meets the performance criteria…”
While no one can doubt the importance of price, there is no mention anywhere in the article about the ability of the polymer to be reused. This simply does not enter into the author’s consciousness. If all of the polymer made by these newly developed methods is used once and then discarded into a dump, this is not seen to be of any importance. This is not considered to be part of the “greenness” equation. All that goes into greenness, according to the article, is whether the raw material comes from petroleum or sugarcane, no matter how soil is degraded or energy is squandered. In the chemical field, discussions of green chemistry have been driven by the most conventional markers (frequently only toxicity and reduced solvent usage) but are not tied in important ways to planetary preservation.
So what path should polyolefin design take to address reusability? Of course, the best way to reuse a plastic part is to reuse it in its highest, complex function. A pump housing should be used in another pump as the same kind of housing, which can only be done if pump design is purposely controlled so that this kind of reuse is built in. But plastic materials are also highly complex molecular assemblies in their own right, aside from the kind of parts they are made into. It is incomparably better to maintain molecular complexity than to break down materials to carbon dioxide or other simple molecules, such as by burning them.
Plastic polymer chains are susceptible to many design parameters. Polyolefins are not necessarily made from long molecular chains that are identical, length after length. They can be combined with other kinds of monomers so that, in each molecule, a long run of polymerized olefins then give way to a run of a completely different kind of monomer (such as styrene or urethanes). The chains are separated into blocks. By combining blocks of different properties and different lengths, the properties of the macroscopic plastic can be fine tuned. This gives us a key way to build in one kind of reusability, as I will explain.
While polyolefin chains are very difficult to break apart chemically, that is not true of all chains. For example, polyester chains can be broken apart fairly easily (there are factories based on this easy reaction and our bodies break apart the ester linkages of starch down to glucose molecules with no trouble). So if we want to be able to break apart polyolefin chains so that we can purify and reuse them, we could add in even a single polyester bond in the middle of a very long polyolefin chain and break the chain apart by the same methods used to break apart polyesters.
The neat part of doing this is that the small presence of the polyester will be so diluted in all the polyolefin that it will change the polyolefin properties almost not at all. Thus we can build in a major reuse capability without changing the plastic properties. Is this a win-win situation or what?
A very important place to apply this kind of modification would be in polyvinyl chloride design. The manufacturers proudly claim that their plastic cannot be reused, though I have never understood why they want to be so definite on this point. Certain environmentalists have proudly repeated this claim because they think it supports their campaign to ban all PVC entirely. But reusability will certainly never be designed into anything while the dump is always welcoming. Smugness will not solve any problems. The highest form of reuse, as formed parts, is already widely available. For example, much PVC ends up in the form of water pipe, which can be trimmed and reused many times. All PVC research needs to be bent toward reuse in every way possible. (see the local article on PVC pipe under Projects).
There are three specially vexing problems affecting plastics reuse:
- The solidity of thermosetting plastics
- The inseparability of laminates
- The molecular breakdown accompanying re-melting and re-extrusion.
Thermosetting: A recent article in Science (18 November 2011 p. 965) reveals some new research into the difference between thermosetting plastics, such as Bakelite, which have rigid forms and do not melt, and thermoplastic plastics such as polyethylene. They are developing ways to make plastics behave like thermosets when that is required, during their useful life, but release their rigid bonds and start to change shape when some kind of new energy is applied, such as light or heat. This could make it possible to have hard, rigid plastics that can nevertheless be induced to change their shapes into new ones when needed.
Laminates: Take a common case of lamination: the plastic sheets used to protect foods such as meats. Let’s assume that we have an outer layer of polyethylene because it is cheap and flexible, even though it has pores that allow oxygen to enter, laminated to an inner layer of a chlorinated PVC or a Saran which has no pores but is more expensive. Can we get the benefits of that sandwich without joining the two sheets together for all time?
For example, could the food be inside a bag of the Saran which was then inside a second bag of polyethylene to make it more portable? Is the lamination used only because it is technologically more “cute” to make a plastic sheet that appears to be a single sheet?
Or could there be a simple way to de-laminate the two different kinds of plastic using light or radiation or a chemical signal or ultrasound or heat?
A Zero Waste principle forbids the joining of two dissimilar materials that cannot be separated or used as is. How would that be applied here?
Chemical and Engineering News (5-19-2014 p. 4) reports a new, hard thermoset based on 4,4′-oxydianiline and paraformaldehyde which sets up hard and yet can be hydrolyzed back to usable monomers by exposure to an acid. As always, it is best to reuse a formed shape without changing that shape but at least the entropic contribution from creating specialized molecules should be preserved.
Breakdown: Despite the naive claims of recyclers, you cannot just keep remelting plastic and reusing it. There is a cost in molecular integrity each time. Can chemists get around this obstacle? Isn’t it better to design parts so they can be reused forever, instead of having to be remelted into new products?
CAUTION: Chemistry ahead! Stop here if your vehicle is not equipped with chemical information devices!
Chemical and Engineering News, August 13, 2012, p. 32. In an article entitled Polymer Healing By Olefin Metathesis, it is announced that a new method has been developed that can heal the cracks that form in certain plastics from use or exposure to stresses. It reports a kind of magic glue, that can be introduced into a crack and then when the sides of the crack are pushed together, the broken polymer chains reform as new, long chains right across the crack, healing it as good as new. The “glue”, which can work in vanishingly small amounts, is actually a catalyst for a certain kind of reaction that the polymer molecules can undergo to re-polymerize. This is just preliminary research at the moment.
Drawbacks so far:
- The catalyst is based on ruthenium, a rare and expensive metal in the platinum family.
- There is no suggestion yet that the plastic part can be dipped into the solution and then compressed to heal all the cracks.
- The polymer must contain an olefin bond, which so far was only tried with cross-linked polybutadiene (a form of rubber). The reaction to be catalyzed is called olefin metathesis.
On the brighter side, many polymers contain olefin bonds and other catalysts may work for other kinds of bonds. Not all repairs consist of replacing broken gears or burned out resistors. This is repair on a molecular level. Stay tuned.
In 2014, a new way to heal holes and cracks in plastics has been developed, in analogy to the way blood clots. Small channels run throughout the plastic filled with monomeric chemicals. When the plastic is broken, the channels are opened to react and a gel fills the cracks or holes, then the gel solidifies and the hole is healed.
IMPLANTABLE MEDICAL DEVICES
A recent research report in Chemical and Engineering News (C&EN Dec 3, 2012 p. 32) explains that siloxane-polyether-urethane block copolymers are widely used in medical devices that are implanted in the human body. It has been found that these plastic copolymers have a route to breakdown at the ambient, wet and warm conditions of the body which consists of hydrolysis of some of the bonds in the plastic.
The article reports that research is being done to find out exactly which bonds are hydrolyzing. The only goal they view is the shortsighted one of making different plastics which last forever under ambient physiologic conditions. Important as far as it goes, but what other potential goals are opened up by such research?
A device that is implanted is not needed forever. Some are removed for cause, while others are found in corpses. Can they be removed for some kind of reuse?
Pacemakers are already harvested to some extent but how about extending that to other devices, such as these plastic ones?
Sure, we want these devices to last for a lifetime, but once that lifetime ends, and we remove them from a corpse, we might have industrial sized pile of plastic which incorporates all manner of other subsidiary devices. Pumps, metals, electronics. Once we understand exactly how hydrolysis takes place, could we create a plastic that hydrolyzes insignificantly at physiologic temperatures but very fast at, say, 200 degrees centigrade in superheated steam? Then the plastic monomers could be separately collected, distilled, purified and reused for new plastics without losing the molecular complexity inhering in the monomers. This is a Zero Waste approach. Redesign the product for a form of reuse that captures the high molecular complexity at least partially. It’s not as good as reusing the entire device for its original function in a new body, but it’s a start.
It is practically taken for granted that plastics are annoying because they will last forever, filling up biological niches as they are eaten by animals and geographic niches as they sit in dumps forever. These notions were developed in the early days of plastic, when hardly any time had elapsed. To anyone paying attention, it was always obvious that sunlight had a destructive effect on plastic. You may have noticed that the ubiquitous, white plastic buckets used for paint, dog food and other products, crack and fall apart after a few years outside. This is ultraviolet attack on the polyethylene chains in the plastic. The same thing happens to polyethylene chairs and tables and, with regrets, to plastic covers for greenhouse frames. However, in 2016, after some time has now passed since the original creation of plastic devices, it is becoming obvious that plastics will self destruct all by themselves. They may get some help from oxygen or trace gases in the air or from timed breakdown or from the migration of some of the fillers and plasticizers out of the body of the plastic. The problem detailed in Scientific American, April 2016, p. 74 et.seq. concerns astronaut spacesuits on display in the Smithsonian museum. Here is an excerpt:
The trouble is the construction material: plastic. Most people think plastics last forever, which makes them a bane to the environment. But although the repeating units of carbon, oxygen, hydrogen and other elements in plastic have a long lifetime, the overall chains – synthetic polymers – do not age well. Light conspires with oxygen and temperature to weaken the bonds that hold the units together. Then chemicals added to plastics to make them bendable or colorful migrate outward, making the surface sticky and wet and perfect for attracting dirt. The polycarbonate spacesuit visor, Young thinks, was leaching out a substance added to make it easier to shape.
Priceless 20th century art is in serious trouble as well. In that era, Andy Warhol, David Hockney and Mark Rothko all used acrylic paint – a plastic polymer popularized in the 1940’s as an alternative to traditional oil paint. Plastic is in fact a building block of much of our recent cultural heritage, including important designer furniture, archival film, crash test dummies, the world’s first Lego pieces and Bakelite jewelry as well as the plastic sculptures made by the pop-art movement. “We now know that objects made of plastic are some of the most vulnerable in museum and gallery collections” says Yvonne Shashoua, a conservation scientist at the National Museum of Denmark and one of the first cultural heritage researchers to study plastic degradation.
The irony is that we moderns thought that we were leaving behind the era of soft, degradable materials and entering a world of digital permanence, where our creations would last forever. Instead, the situations seems to be exactly the reverse where some 5000 year old cellulose papyri can still be read, oil paintings can still be viewed and stone lasts for many millenia while floppy discs from ten years ago are already unreadable.