MSS Optical sorting machine

Humans are currently producing about 100 pounds of plastic per person per year on this planet (~250 pounds the US) and at the current rate of increase that number will be at ~125 pounds by 2030. Actually, recycling is not the desired outcome from a waste reduction perspective; the hierarchy is reduce, reuse, recycle. Reduction and reuse, for the most part, are individual decisions while the latter also requires infrastructure which is why I think the latter gets more scrutiny in the media.   

After a plastic is used, four fates are possible. If the plastic is collected, then the plastic can either be recycled, landfilled or burned. The latter is done usually for fuel value, i.e. plastic replaces a fossil fuel (the structure of most plastic is similar to oil and the energy value is similar). The fourth of course is that either prior to collection (or even after collection), the plastic migrates into the environment. In developed countries, such migration is rare but in developing countries, especially in Southeast Asia, migration is much more common. Approximately 90% of all ocean plastic comes from 10 rivers, of which 6 are either entirely or significantly in China and the remaining 4 are the Ganges, Nile, Mekong and Niger.  

All plastic that is recycled today involves melting a previously used plastic and reforming it into a different shape. The amount of plastics recycled varies widely from country to country, and even among different locations within a country. Given that collection and transportation costs are a significant part of the total recycling cost, dense populations have significant advantages in terms of recycling. The figure below shows data for Europe; in Japan 23% is recycled, 7% is landfilled, 9% is burned and not used as fuel, while the rest (61%) is burned and used for fuel. For the US, 9% is recycled, 16% is burned for fuel and 75% is landfilled.

From Plastics–the Facts 2018, by Plastics Europe

Population density explains some of the reason for higher recycling rates in Europe and Japan, but probably more important is the governmental support for such programs. Also, the figure above is consistent with the hypothesis that there is a limit to the recycling rate for plastics of 30-40%, which I believe is a correct interpretation of current conditions.  There are both scientific and consumer preference reasons for this limit.

• Recycled plastic parts will be much less brittle if plastics are separated by type. The technology exists today to do this in a relatively cost effective way, although since margins are small in this industry and the equipment is expensive, manufacturers are reluctant to purchase this equipment. However, even with perfect feedstock the perfectly separated product will have properties that are a blend of all the properties of the same resin from different manufacturers (or from the same manufacturer with slightly different specifications optimized for the particular bottle shape). So, for example, if you recycled clear plastic water bottles, it’s not certain that you could even produce clear plastic water bottles the second time. Making fiber is much more forgiving than making bottles, which is why recycled polyester often ends up in clothes (50% of all clothing material today is polyester).   Feedstock variability is a large issue as well in recycling.

• Because plastics are many different colors, what results from recycling them all together usually does not have an appealing color. Technologies exist for sorting colors, but mixing various shades of blue, for example, is not guaranteed to result in an appealing color. Outside of clear plastic beverage bottles, which are easily segregated, black is really the only option for color consistency. Currently, the recycled content in colored bottles is often in an inside layer (i.e. you can’t see it!).
  
  
• Other problems with recycling include the following:
  • Ink is printed on ~10% of plastic products which, during recycling, is incorporated into the product which in turn increases brittleness significantly. Clear beverage bottles have a label which is removable for this reason.
  • Impurities (food residue, dirt etc.) are another issue: without some significant cleaning, contaminated product is pretty useless for recycling. In principle neither ink nor impurities are really an issue; work done in my lab shows that you can wash plastics in much the same manner as washing clothes. However, there is an environmental and cost impact to this action. 
  • Multilayer or laminated films contain more than one type of polymer or polymer laminated to a metal (think shiny!). I am not sure how much of plastic is multilayer; my best guess is 5-10%.  

To increase beyond the 30-40% ceiling in recycling rates in my opinion requires significantly more government intervention than is practiced today. 

Technology could possibly change the calculation. Many research groups and companies are looking at depolymerizing the plastic to the base monomer (see my previous post). I personally am skeptical that this will ever compete with melting and reforming from an economic viewpoint.

Single use plastic bags

I would like to conclude with two points. First, as I said above, as a waste reduction strategy, recycling comes after reuse. But reuse is not always simple. For example a resuable plastic bag has about 12 times as much plastic as a single-use bag, so the bag needs to be used more than 12 times before disposal.  A recent study showed that in California, where single-use grocery bags have been banned, 12 million of the 40 million pounds of disposable plastic bag reduction has been offset by an increase in garbage bag purchases. I have heard anecdotally (though can’t find any proof) that more plastics are sold in California than before because people do not reuse the reusable bags enough times before disposal. The author of the California study clearly points out in another article that she did not consider increased sales of reusable bags. In my own house, I have more reusable bags than I need, and have at times thrown them out (I’m not proud of that, but it is true!).

Second, from a carbon footprint perspective, plastics are a beneficial material compared to the alternatives in most cases. Consider plastic bags vs. paper bags. A single use plastic bag has a 2-3 times smaller carbon footprint then a single-use paper bag. We of course have to weigh the fact that a paper bag will rapidly degrade in water and presumably is little to no hazard to marine life, which cannot be said for plastics that enter rivers, streams and oceans. However paper bags are much more environmentally unfriendly in terms of their production (considering water use, and chemicals entering the environment). Being a good environmental steward requires choices like this sometimes. Admittedly, the carbon impacts of paper vs. plastic bags are both miniscule compared to the other sources of carbon.  

Something that is not miniscule is the reduction of greenhouse gas emissions (GHG) because of the use of plastics in cars. A rule of thumb is that for every 1% reduction in vehicle weight, fuel consumption drops by 0.5%. Estimates of how much plastic could actually compose a car vary widely, but a reduction in weight of 50% due to the replacement of metal and glass with plastic is certainly a reasonable guess, which would reduce gasoline usage of a gasoline powered car by ~25%. Since GHG from light duty vehicles is about 18% of the total emissions in the US, one could easily expect a reduction of 5% in GHG by putting more plastics in cars (as I noted in my previous post, changing to electric cars, assuming no significant increase in nuclear power, will likely not have a large effect on GHG emissions in the foreseeable future). There are also significant weight savings in all other forms of transportations via the use of lighter weight plastics vs. metals.

Plastics are an incredibly important material in modern society and I hope I have made the point that in some cases plastics might be an environmentally better alternative. However, recycling rates in the United States are far lower than in other developed countries, which I view as somewhat embarrassing. Finally though, to have the majority of plastics recycled will require significant changes on the part of manufacturers, consumers and government.