Materials in Today's World

Recycling Polymers


One way of classifying polymers is to break them up into two classes. The two classes of polymers are thermoplastic polymers and thermosetting polymers. The basic property that separates a thermoplastic polymer from a thermosetting polymer is the polymer’s response to being heated. When the thermoplastic polymer is heated, it melts, softens, and can be reformed when cooled. When the thermosetting polymer is heated, it hardens and cannot be reformed and stays hard when cooled. We will learn much more about each of these two classes of polymers and the reasons for their defining properties later in our lesson on polymer structures.

Since thermoplastic polymers can be melted and reformed, they are easily recycled. However, their properties do degrade with each reuse. Thermosetting polymers are much more difficult to recycle. Some of them can be ground up and used as filler for other processes, and, on a case-by-case basis, some can be processed to be broken down into their underlying base units which can be reused. Another approach to reducing the amount of plastic that ends up in our landfills is the development of biodegradable plastic. The idea here is that plastic can be made to breakdown (be compostable). In addition, bioplastics often come from renewable raw materials. But this leads to an ethical issue: do you use the available arable land for plastic or food production?

Now, please watch the following video (4:38) on plastics and biodegradable plastics which summarizes some of the issues around plastic recycling and bioplastics as discussed in your e-book and this website.

To Watch

Click for transcript of Problems with Plastics and Biodegradable Plastics.

In the previous video, Limits to Recycling, we looked at the overall limits to recycling of materials. We touched on plastics in that video. You may be aware that the biggest problem in recycling plastics is they are not biodegradable, so that soft drinks bottle or your wrappers that you just throw away and not biodegradable. This means the fate of most plastics is to go into landfill sites or our oceans where it will take hundreds possibly thousands of years for them to degrade. You may say what about recycling? Briefly, the problems with recycling plastics are they have to be manually sorted which is a labor and energy intensive process. Mixed polymers are found in a range materials so cannot be recycled and are often dumped. Once a plastic has been recycled once it can be come downcycled whereby its quality is decreased. The fact is that plastic recycling rates are far below other recycling rates, but the industry has improved in recent years.

Worldwide seven groups of plastic polymers have been given a plastic identification code that people use for recycling. These you can find the most packaging or plastic materials different types of plastics will be recycled by different municipalities even by different countries in some instances. Generally it's difficult to recycle plastics with numbers one and two. Plastics in these higher numbers are known as rigid plastics. So, the disposal of plastics is a tricky one. Other than reducing their use recycling or reusing them what else do you think might be done to dispose of them? Pause the video and continue when you think you have an answer.

One way is that the plastic can be incinerated or burned. Whilst this produces useful energy, some plastics contain polymers that are halogenated When these polymers are burnt. For instance, PVC, toxic fumes like hydrochloric acid are released. This can cause severe respiratory distress and so is hugely problematic. In addition, burning of polymers produces CO2. The greenhouse gas also contributes to global warming. This is because about 10% of the oil is used as petrochemicals. If we then use the polymers again as a fuel for energy production then at least we're using the oil twice. You may have heard the idea of using bio plastics as a solution, but these two can have problems. What do you think bioplastic is? Pause the video and resume when you have an answer.

Bioplastic is a biodegradable plastic. This means it's compostable and it can be broken down by the result of bacterial action when it is disposed of. Whilst this is a benefit, there are still issues with bio plastics that need solving. Bioplastics often come from renewable raw materials like starch, maize, cellulose, and poly lactic acids. These are plant derived materials and so the ethical question arises is where the land should be used to grow crops or bioplastic manufacture instead of crops for food? Originally we said that bio plastics are readily compostable, however, bio plastics are not nearly as readily compostable as regular plant material. If you toss a bio plastic fork into your compost and assume it will be dirt in a few months you'll be disappointed. Whilst a bioplastic fork is compostable it requires high intensity high heat commercial composting for that to happen quickly.

In summary, plastic disposal has a number of issues with regards to landfills, recycling, and incineration. However, bioplastics so slightly better do not present themselves as the better alternative because there are issues of land usage, the quality of the bio plastic, and how it is composted. The best alternative is to reduce our usage and disposal of plastics all together.

In the previous video, incineration of waste was discussed. Incineration leads to a huge volume reduction of waste, which results in less waste ending up in the landfill. Waste in the landfill is the least environmentally friendly option. However, incineration typically results in less recycling, which would be a more efficient use of recyclable material than incinerating it. This reduction of recycling due to incineration is considered the major disadvantage of incineration. Although an important concern with incineration is the production of toxins, with proper technology these toxins can be managed. A segment of the video for this week, Making Stuff: Cleaner, discusses burning waste to create electricity. Please watch the following short video (4:40) which discusses burning waste to create electricity as well as the issues regarding incineration discussed above.

To Watch

Click for transcript of Incineration of Waste and Electricity.

In this video we're going to consider the advantages and the disadvantages to waste incineration. What do you think are the advantages of burning waste instead of committing it to landfill? Pause the video and continue when you have an idea or some ideas.

If the waste is not burned, then it is likely to end up in a landfill site which is considered to be the least environmentally friendly option. Incineration could result in a reduction in waste volume of around ninety percent and this could be particularly important for small cities where space is scarce and landfill is not an option. Odors and rodents are present in other methods, are not a problem when using incineration as a garbage disposal method. Nearly all of the waste that is burned could be used to generate electricity in what is called energy-from-waste schemes where households and industries could benefit from the electricity or heat produced. The electricity generated could help to pay for the startup costs of the incinerator. Additionally, the steam produced from incineration presents itself as a cost-saving energy source if recycled. Ash produced from these incinerators could be used in the construction and road building industries. In addition, metals could be extracted from the ash and they could also be used in steel industries. The key advantage, however, is that the production of electricity and thermal energy from waste enables us to conserve conventional sources of energy such as fossil fuels? Now, what about the disadvantages of incineration? Again pause the video and think about what these could be. Resume when you have an idea or some ideas.

The main disadvantage surrounds potential pollutants found in the ash left in the incinerator and those that could be emitted from the chimney. These include dioxins, acid gases, nitrogen oxide, heavy metals, and particulates. As you may have heard from other videos, these are airborne particles which are small enough to get into the lungs of humans. They cause and aggravate respiratory problems such as asthma; however, it is the dioxins resulting from incomplete combustion contained in the gases from the chimneys that attract the most concern. This is because they are suspected of causing cancer. In addition, these emissions could be distributed through the food chains and accumulate over a long period of time impacting on both ecosystems and human health. As a result many people are opposed to having incinerators built in or near their communities.

Another disadvantage is the initial startup costs to build a waste incinerator for the production of electricity and may not be seen as a viable alternative. Where cheaper waste disposal methods exist, once built the maintenance of the machinery could be costly. The biggest disadvantage however, is that if waste is incinerated without first being sorted this means we are incinerating waste that could have been recycled.

So in summary, incineration of wastes presents numerous advantages. One, the energy is used the heat or to produce electricity for homes in industry. Two, this method reduces the need for fossil fuel consumption. Three, the ash produce can be recycled for metals used in the steel industry, and finally four, this method reduces the volume of waste produced. There are also disadvantages. One, incinerators are expensive to build and maintain. Two, pollutants in the ash and emissions from incinerators are harmful to health and ecosystems and finally three, waste that could be recycled will instead be incinerated if not properly sorted.

Lastly, please watch the following video (5:40) on the recycling of paper, which touches on several themes of this lesson including sustainability, downcycling, and green design principles.

To Watch

Click for transcript of Recycling Paper.

Look around you. There are so many paper products from the sheets of paper that you write on to the pages of your textbook - to egg cartons, to newspapers, to certain grocery bags - cardboard boxes, and the list continues. The important fact here is that paper is an essential part of our lives. You may know that paper usually comes from trees. They must first be chopped down and then subjected to an industrial process called pulping which removes lignin, the polymer that gives trees their strong hard structure. Obviously the paper production process has led to mass deforestation. The main issue here is that the trees are not being replanted, or if they are being replanted they will not grow fast enough to replace the trees that are being cut. This is a serious environmental issue because it destroys habitats, disrupts food chains and food webs, is a severe eyesore, and indirectly leads to increased carbon dioxide content in the atmosphere. It could also lead to erosion of hillsides with the consequence of flooding. An obvious solution is to recycle these paper products. In this lesson, we will learn about recycling paper and the advantages and disadvantages to this industrial process.

Paper is in fact layers of cross-linked cellulose fibers. Recall that cellulose is a polymer of glucose monomer units. If you tear a piece of paper and look closely, you will see fine hairs sticking out. These fine hairs are cellulose fibers. When paper products are recycled the cellulose furs are merely pulled apart and rearranged into new paper products. In some countries, paper recycling is highly encouraged by having specialized waste containers just for paper waste. The paper waste is then collected and separated according to their properties. Sheets of paper are recycled with other sheets of paper and cardboard boxes are recycled with other cardboard boxes.

Now, right before the paper can be recycled, any ink needs to be removed. This process is called de-inking. Most inks are petroleum-based inks which are derived from crude oil products and can be difficult to remove. Currently in development are soy-based inks which are derived from a renewable resource and makes it easier for the paper to be de-inked. Metal parts such as staples and paper clips need to be removed with a magnet and glues are removed with a chemical wash. Bleach is sometimes added to whiten the slurry and therefore, the resulting paper product will be brighter. The cellulose fibers in the slurry are mixed with new pulp and then allowed to settle on a gauze and rolled to form the new sheets of paper. This step also removes any excess water from the slurry before it is air dried.

The main disadvantage with recycling paper is that it can only be done a few times recycled paper is usually used to make kitchen rolls, toilet paper, and packaging materials. Each time a paper product is recycled the cellulose fibers become shorter thereby rendering the recycled paper product a little bit less useful than the original. However, this can be used for another purpose. Can you remember what cellulase is? Hint: it is an enzyme. Please pause the lesson to think about this and resume when you are ready.

Cellulase is the enzyme that breaks cellulose apart into its glucose monomers. Currently the industrial usage of cellulase is to break down cellulose into glucose. The glucose obtained can then be fermented by certain bacteria to produce ethanol, which can then be used as a biofuel.

In summary, recycling paper products help to reduce the rate of deforestation and consumes a lot less energy. Although paper can only be recycled a few times, the cellulose fibers can be broken apart into glucose monomers which can then be used for other environmentally friendly applications.