Uses of Polymers, Plastic Waste and Its Management

1Why ban something so useful?

Since 2008 it has been against the law in Rwanda to make, import, use or sell polythene carry bags, and in 2019 the ban was widened to single-use plastic items. Yet plastics are cheap, light, strong, waterproof and easy to shape, and hospitals, farms, builders and electricians depend on them. The problem is not what plastics do while we use them, but what happens afterwards: the same strong C–C chains that make them last for years in use make them last for centuries as litter.

2Uses: matching the polymer to the job

Table 4.1: Uses of common polymers and the properties behind them

3Why plastics persist, and the damage they do

Addition polymers such as PE, PP and PVC have only C–C and C–H bonds in their chains. These are strong, non-polar and water-repelling, and microorganisms have no enzymes that attack them, so the polymer is non-biodegradable. Sunlight and heat only break it into smaller and smaller pieces. The damage includes:

• Blocked drains and floods. Bags and bottles clog gutters and culverts, so heavy rain floods streets, and stagnant water behind the blockage lets mosquitoes breed.
• Soil and farms. Buried film stops water and air reaching roots. Cattle and goats that eat bags can die from blocked stomachs.
• Water and food chains. Fish and birds swallow fragments; microplastics are now found in rivers, lakes, drinking water and food.
• Open burning. Burning plastic at low temperature gives carbon monoxide, soot and toxic organic compounds, including dioxins. PVC also releases hydrogen chloride, and nitrogen-containing polymers such as nylon and polyurethane foams can release hydrogen cyanide.

The scale is large. The OECD's Global Plastics Outlook (2022) estimated that the world made 460 million tonnes of plastics in 2019 and threw away 353 million tonnes. Only 9 % of that waste was recycled; 19 % was incinerated, about half went to landfill, and 22 % was mismanaged: dumped, burned in the open or leaked into the environment.

4Rwanda's law on plastics

Rwanda first banned polythene bags with Law N° 57 of 2008 (10 September 2008). This was replaced by Law N° 17 of 2019 of 10 August 2019 relating to the prohibition of manufacturing, importation, use and sale of plastic carry bags and single-use plastic items, which came into force when it was published on 23 September 2019. Its main provisions are:

• the manufacturing, use, importation or sale of plastic carry bags and single-use plastic items is prohibited (Article 3). A single-use plastic item is defined as one designed to be used once before it is thrown away or recycled, and the definition includes oxo-degradable plastics (Article 2);
• home-compostable plastic items and woven polypropylene (the material of grain and cement sacks) may be made, imported, used or sold with prior authorisation from the environmental authority, and other uses need an exceptional authorisation (Article 4). In practice these authorisations are issued by the Rwanda Environment Management Authority (REMA);
• imported goods packaged in plastic pay an environmental levy (Article 5);
• every manufacturer, wholesaler or retailer of plastic carry bags or single-use items must collect and separate the used items and hand them to recycling plants (Article 6);
• anyone who dumps plastic bag waste or other single-use plastic items in an unauthorised place pays an administrative fine of 50,000 Frw and must remove the waste (Article 13).

Figure 4.1 The waste hierarchy. The best option is to use less; the worst is to bury waste. Recycling is in the middle because it still uses energy and each cycle shortens the chains.

5Managing old polymer materials

Sorting. Recycling needs a single polymer. Many items carry a resin identification code, a number from 1 to 7 inside a triangle of arrows (Table 4.2). Shredded mixtures can be separated by density: in water, PP and PE float, while PET, PVC and polystyrene sink.

Table 4.2: Resin identification codes

Mechanical recycling works only for thermoplastics: they are washed, shredded, melted and extruded into new pellets. Each cycle breaks some chains and adds contaminants, so recycled plastic is often "downcycled" into lower-grade goods, such as fleece fibre from bottles, or mixed with new polymer. Thermosets and vulcanised rubber cannot be remelted; they are ground up and used as fillers.

Chemical recycling breaks polymers down. Condensation polymers can be taken back to monomers by hydrolysis, or by reaction with methanol or a diol (Lesson 2); PET, for example, gives benzene-1,4-dicarboxylic acid (or its ester) and ethane-1,2-diol, which can be purified and polymerised again. Addition polymers such as PE and PP can be pyrolysed: heated to about 400 to 500 °C without air, they crack into a mixture of shorter alkanes and alkenes used as fuel or chemical feedstock.

Energy recovery. Plastics are made from crude oil and burn with a high energy release. In a modern incinerator they are burned completely at high temperature, the heat raises steam for electricity, and the acidic gases are removed by scrubbing, for example $\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{g})+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}(\mathrm{a}\mathrm{q})\to \mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\backslash mathrm\{HCl(g)\; +\; NaOH(aq)\; \backslash rightarrow\; NaCl(aq)\; +\; H\_2O(l)\}$. This is very different from burning in the open.

Landfill is the last resort: an engineered landfill is lined to stop leaks, but the plastic stays there.

Better materials. Biodegradable polymers include PLA (Lesson 2), PHB, poly(3-hydroxybutanoate), a polyester made by bacteria, and starch-based plastics. Their ester links can be hydrolysed and attacked by microorganisms. "Oxo-degradable" plastics are a trap: additives make ordinary PE crumble in sunlight, but the fragments do not biodegrade; they become microplastics. This is why Rwanda's 2019 law counts them as single-use plastics.