Plastics and Recycling

3D printing and plastics go hand-in-hand.

3D printing benefits hugely from the inherent properties of thermoplastics - their relative low cost, mechanical functionality, resistance and usability. Those little spools (or not) of plastic are currently the life force of the industry.

Such an intimate relationship means that 3D printing must also share in the responsibility of the problems that accompany plastics. Chief among these is the question of environment and sustainability. Being such a future-oriented industry, it's a factor that 3D printing must consider.

In this article we want to better understand what the plastic recycling process currently looks like and some of the issues surrounding it.

The Situation - General Plastic Recycling

The data available on plastic recycling is surprisingly limited. In 2014, global plastic recycling rates were estimated to be 18%, with a further 24% being incinerated.

  • Europe: 30% recycle rate (2014).
  • Chine: 25% recycle rate (2014).
  • America: 9% recycle rate (2014.

Whilst these figures are much better than 20 years ago, they are relatively low compared to other 'disposable' commodities. Paper, for example, has a global average of 58% and highs of 75% in some countries (Cepi.org, 2015).

The rates of plastic recycling vary greatly depending on a number of factors such as geography, plastic type and application.

Before we can understand why the rate of plastic recycling is so varied and often low, we first need to look at the recycling process itself.

Processing

Before plastic can be recycled, it must pass through 5 different stages of processing.

  • Stage 1: Sorting - Each plastic item must be separated by its type and sometimes its colour too. Separation used to happen by using the Plastic Resin Identification Codes. Nowadays, automated sorting systems are far more common, usually involving density tests to identify and separate the different plastics. Find out how you can identify plastics at home.

  • Stage 2: Washing - Impurities such as labels and adhesives must be removed or they will impair the quality of the final plastic product. This is achieved with thorough washing.

  • Stage 3: Shredding - The sorted and washed plastic is then run through what is essentially a pumped-up paper shredder! This cuts and grinds the plastic down into small pellets.

  • Stage 4: Classification - Once shredded, the plastic pellets are tested to determine their quality.

  • Stage 5: Recycling

There are multiple ways in which plastic can be recycled that vary in complexity and efficiency. Broadly speaking, recycling methods fall under 4 categories.

Primary Mechanical processing into a product that has similar properties.
Secondary Mechanical processing into a product that has lower properties.
Tertiary The plastic is de-polymerized and its chemical constituents are recovered. This process is often called chemical or feedstock recycling. Tertiary recycling also includes composting of biodegradable plastics.
Quaternary Energy recovery - often through pyrolysis (high temperature decomposition). Generally only used for plastics that cannot be recycled another way as this process produces products that must be safely removed, such as nitrogen oxides. Pyrolysis also produces carbon dioxide.

Issues

Contaminants

One of the key issues in the plastic recycling process is that many plastic products use other additives or contaminants such as inks, paper, glue and other adhesives. The problem with these contaminants is that they reduce the quality of the recycled plastic, therefore reducing it's economic value. Plastics that are not economically viable to recycle are often moved into the less preferable Quaternary recycling stream.

Mixed Polymers

Many plastic products also use multiple polymers in the same product. It is essential that these different polymers are separated from each other during processing. If they are not sorted, when they are melted they will form separate phases or layers due to their varying densities (think oil in water). For a brief look at polymer densities check out the table in Identifying Unknown Filaments. When solidified, the interface between the two polymers is a significant structural weak point.

It is also important to consider the different processing temperatures for different polymers. For example, PVC has a lower melting temperature than PET. If even a small amount of PVC gets into a PET recycling stream, when it is subjected to the higher PET melting temperature it will produce hydrochloric acid gas. On the other hand, PET in a PVC recycling stream will not melt and will leave solid lumps in the recycled plastic.

High cost, low revenue

Whilst Tertiary recycling is more efficient in the sense that it can directly recover the usable polymers which can then be used to produce plastics or other chemicals, it is rarely used due to its economic inefficiency. This is due to the low prices of the chemical feedstock that it produces and the high costs of producing the feedstock from the polymerized plastics! Some polymers, such as PET, can be broken down under easier conditions so are more economically viable for Tertiary recycling.

Waste Sorting

Another less technical issue that exists is due to the way plastics are used. Disposable packaging represents a significant portion of plastic usage. Due to it's disposable nature, the plastic is not always separated from the general waste stream at the source (i.e. homes, offices etc.). This is particularly prevalent in food waste. The issue here lies both with the education of the user and also with the heavy use of plastic wrappings (Think about the vegetable section of your supermarket).

The United States Environment Protection Agency (APA) defines containers and packaging as products that will be discarded in the same year as the products they contain are purchased. The EPA and the American Chemistry Council published statistics showing that in 2015, the United States generated approximately 14.7 million US tons of plastic containers and packaging (1 US ton = ~907kg). They also estimated that approximately 10 million US tons of this went to landfill.

Statistics from EPA data sheets (Epa.gov). The EPA also has a great table showing plastic container generation over time on their Facts and Figures page (US EPA).

Biodegradable Plastics

It's worth mentioning biodegradable plastics on their own, as there as significant differences in how they are recycled. Before that, make sure you know the difference between biodegradable and compostable plastics.

The popularity of PLA in 3D printing is particularly significant as biodegradable plastics get around some of the issues associated with normal plastic usage.

One of the issues that biodegradable plastics may address quite well is that they need not be separated from the organic waste stream. Very importantly however, local recycling facilities need to be equipped with the correct technology and handling systems to process biodegradable plastic.

Another benefit of biodegradable plastic is the fact that it is bio-based, reducing the number of undesirable substances produced during its recycling. The exception is the methane that is produced during anaerobic (no oxygen) decomposition however this can be recaptured for energy use. Alternatively, waste facilities can include biodegradable plastics in aerobic composting.

A major issue currently being faced by bio-plastic use is the sheer amount of bio-mass that would be required to replace a significant portion of the plastic that is currently being used. Bio-mass production is currently far from sufficient.

Despite the issues, an increase in the use of biodegradable plastics is desirable as long as they work well with current waste management systems (e.g. labelling, collection etc.).

The purpose of this article was to have a general understanding of the current climate of plastic recycling, how it works and what issues are facing it. In the next few articles, we'll look at some of the initiatives that are being used to tackle plastic recycling!

Useful Resources

Cepi.org. (2018). Global Forest and Paper Industry Releases Policy Statement on Paper Recycling | CEPI - CONFEDERATION OF EUROPEAN PAPER INDUSTRIES. [online] Available at: http://www.cepi.org/news/global-forest-and-paper-industry-releases-policy-statement-paper-recycling

Epa.gov. (2018). [online] Available at: https://www.epa.gov/sites/production/files/2018-07/documents/2015_smm_msw_factsheet_07242018_fnl_508_002.pdf

Ford, S. and Despeisse, M. (2016). Additive manufacturing and sustainability: an exploratory study of the advantages and challenges. Journal of Cleaner Production, 137, pp.1573-1587.

Geyer, R., Jambeck, J. and Law, K. (2017). Production, use, and fate of all plastics ever made. Science Advances, 3(7), p.e1700782.

Hopewell, J., Dvorak, R. and Kosior, E. (2009). Plastics recycling: challenges and opportunities. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1526), pp.2115-2126.

Norcalcompactors.net. (2018). Plastic Recycling - Processes, Stages, and Benefits. [online] Available at: http://www.norcalcompactors.net/processes-stages-benefits-plastic-recycling/

Pcn.org. (2018). Plastics Consultancy Network - Recycling. [online] Available at: http://www.pcn.org/Technical%20Notes%20-%20Recycle1.html

US EPA. (2018). Containers and Packaging: Product-Specific Data | US EPA. [online] Available at: https://www.epa.gov/facts-and-figures-about-materials-waste-and-recycling/containers-and-packaging-product-specific-data#PlasticC&P