Know your Plastics Series: Polymers

Plastic is everywhere! Plastic is in everything!

Plastic has pervaded every aspect of human existence and is as commonplace as the sand on the seashore.

Its benefits are numerous and have been discussed in some detail in the preceding article in the Know Your Plastics Series (Click here to read). However, its major disadvantage is improper disposal and recycling of plastics when discarded. To recycle plastic material more effectively and efficiently, one must understand the components and constituents of each plastic-type, known as polymers

What is a Polymer?

Hermann Staudinger, an organic chemistry professor at the University of Applied Sciences in Zurich in the 1920s is the “Father of Modern Polymer development”. His discoveries in macromolecular chemistry earned him the 1953 Nobel Prize in Chemistry.

Monomers in a Polymer

A polymer is any natural or synthetic substance made of multiple, identical, repetitive units of smaller molecules, AKA monomers, which are bonded together with heat and pressure in a polymerization process that creates long chains of macromolecules. The resultant molecular weight or mass of a polymer is usually quite large and this is why a polymer is sometimes referred to by the term ‘high polymer’ or ‘macromolecule’.

Depending on the type of molecules bonded and the method of bonding, polymers can contain very distinct properties. For instance, while rubber and wood are natural polymers that contain isoprene, a simple hydrocarbon, protein is a natural amino acid-nucleic acid polymer composed of complex molecules of nitrogen bases, sugars and phosphoric acid.

Other natural polymers include carbohydrates, wool, silk, leather, cotton and all the solid parts of plants such as cellulose, lignin and various resins. Polymers also form the basis of inorganic materials of minerals such as graphite, quartz, diamond and feldspar.

Synthetic polymers include polyethylene, polypropylene, polybutadiene, polyisoprene, polychloroprene, polyvinyl chloride, polystyrene and polyurethanes. They are used to produce materials like rubber, plastic, spandex, glass, concrete and paper. Did you notice that the scientific names all contain the suffix “poly’” thereby reinforcing the multiplicity of the “poly”-mer? #smiles

Disclaimer: I do not have a background in chemistry. That said, an attempt to simplify every tongue-biting scientific term may not be expedient. For this series, the focus will remain on plastic polymers.

Structure and Types of Polymer

People often use the terms plastic and polymer interchangeably; this is incorrect. Hence, the saying “All plastics are polymers but, not all polymers are plastic” A plastic material is essentially a compound made up of a polymer and additives (either modifiers or stabilizers) that can be moulded or shaped into useful products. The primary difference between plastics and rubbers (elastomers) is the high level of stiffness in plastics which leads to a deficiency in reverse elasticity.

Pictoral Structure of Homopolymers and Copolymers

Polymers have two basic structures:

1. Homopolymer – When the monomers in a polymer are the same or based on one type of monomer, the polymer is called a homopolymer. For instance, polypropylene and polystyrene are polymers with a long chain (backbone) of carbon-to-carbon atoms. An example of plastic with a homopolymer structure is a plastic milk jug.

2. Copolymer – The polymers here have two or more monomers. For instance, when long chains of carbon-to-carbon atoms contain oxygen and nitrogen at intervals, they can form polymers like polyesters, nylons etc. It is necessary for products that will be used for harsh content like bottles for laundry detergent. The more monomer combinations there are in a polymer, the greater the variation in properties it will possess. These variations will create a more customized plastic product.

Since different plastic structures are suited to different design and performance applications, one can deduce that not one single plastic is suited to all human needs. This deduction leads us to the different types of polymers used in plastic.

Types of Polymers

Plastic polymers are of two types. They are thermoset plastics (thermosets) and thermoplastics.

Differences between Thermosets and Thermoplastics.

Thermosets

Imagine the principle guiding raw and cooked eggs: Once the heat is applied (boiling, frying or poaching), the egg cannot return to its initial state. The same applies to a thermoset.

When thermoset resins are processed for the first time, they are heated and compressed in a chemical reaction that forms a dense, infusible network with strong cross-links that bind the long molecular chains together. This network essentially becomes one large molecule that is hard, not soluble and cannot be remoulded with heat once the product has been cured or set into the finished product. Any heat applied after curing may result in a total breakdown or decomposition of the polymer. Common thermosets include:

- Tires of automobiles

- The insulating materials used in buildings

- Mattresses and cushions

- Bathtubs and shower stalls

- The non-stick coating on cooking pots and pans

- The coatings for electrical devices

- Hard-plastic crockery

- The epoxy resin used in heavy-duty adhesives and wood fillers.

Strength and durability are the two critical features of thermosets. They are why thermosets are extensively applied in heavy-duty industries, especially in construction and automobile manufacture.

Thermoplastics

When thermal energy (heat) makes something soft and flexible AKA plastic, that item is thermoplastic. This characteristic is possible because thermoplastics contain long polymer molecules held together by weak, secondary bonds which fall apart when exposed to heat and are quickly formed again when the heat is absent. This ease in melting and recycling is what separates thermoplastics from thermosets. Examples include:

- Plastic bottles, jugs, ropes and sheets of polythene

- Packaging material (especially the crumbly while ones)

- Bank cards (credit and debit cards)

- Headlamps of cars and motorbikes

- Nylon stockings

- External prostheses

- Hard plastic windows

- Toothbrushes

- Conventional swimwear – shorts, trunks etc.

- Umbrellas

- Toys etc.

I can say with a great degree of certainty that the vast majority of plastics that you have ever come in contact with are of the thermoplastic type. It is possible because the ease with which this type of plastic can be repeatedly heated and reformed into a wide range of applications makes the process of manufacture and recycling very easy.

Plastic Disposal: Recycling and Resource Recovery

In the past, the favoured method of general solid waste disposal was sanitary landfills. In more recent times, this method has resulted in several land-use concerns that precipitated the need to consider recycling disposed materials for reuse and other benefits.

Plastic recycling is a bit complex.

The varying structures, types and customizations of plastic polymers mean that there are different types of plastic. For recycling to be effective, plastic types must be identified, segregated or sorted and gathered individually.

Currently, seven types of plastic are known. They are:

1. Polyethylene Terephthalate with acronym PETE or PET.

2. High-Density Polyethylene with acronym HDPE or PE-HD.

3. Polyvinyl Chloride with acronym V or PVC.

4. Low-Density Polyethylene with acronym LDPE or PE-LD.

5. Polypropylene with acronym PP.

6. Polystyrene or Styrofoam with acronym PS.

7. Other plastics or resins with the acronym OTHER or O.

In the coming weeks, we will be highlighting each type - what they look like, their differences, what products they produce and if and how they are recycled.