Question

What are Polymers? How do they tie into recycling plastics?

Please give lots of details

Answer 1

Let’s start with an introduction to polymers; the word ‘polymer’ has been derived from two Greek words: poly which means many and ‘mer’ means unit. So, we can define polymer as something which is composed of many repeating units. In reality, polymer is defined as very large molecules that have high molecular mass and so they are also referred to as macromolecules. Macromolecules are ones which are formed by joining of repeating structural units and this repetition takes place on a very large scale.

Let’s move on to classification of polymers. Polymers cannot be classified under one category because of its several usages and different behaviors and structures. We can therefore classify it on the basis of the following considerations:

Classification of polymers based on origin

There are three types of classification under this category.

1. Natural polymers: These are polymers which occur naturally and are found in plants and animals. For example: proteins, starch, cellulose and rubber. To add up, we also have biodegradable polymers which are called bio polymers; this will be discussed here a little later.
2. Semi – synthetic polymers: They are derived from naturally occurring polymers and undergo further chemical modification.
3. Synthetic polymers: These are man-made polymers. Plastic is the most common and widely used synthetic polymer. It is used in industries and various daily products.

Classification of polymers based on structure

This category has the following classifications:

1. Linear polymers: The structure of polymers containing long and straight chains fall in this category. PVC, i.e. poly-vinyl chloride is largely used for making pipes and electric cables, is an example of a linear polymer.
2. Branched chain polymers: When linear chains of a polymer form branches ,then such polymers are categorized as branched chain polymers. Low-density polythene is an example of branched chain polymers.
3. Cross-linked or network polymers: These polymers are composed of bi-functional and tri-functional monomers. They have a stronger covalent bond as compared to various linear polymers. Bakelite and melamine are examples in this category.

We have classified the general polymers, but missed out on one and that is bio polymers. Bio polymers, as the name suggests is obtained from the living organisms. In a broad sense we can also say that these are biodegradable polymers. There is a difference between the synthetic polymers and the bio polymers; the structure is very well defined in case of bio polymers. Bio polymers include various bio molecules for instance carbohydrates and proteins. These molecules fold into characteristic shapes as found in protein molecules where folding occurs.

High-temperature polymers

The polymers which are stable at high temperature are known as high-temperature polymers. The molecular weight of these polymers is high so that they are not destroyed at high temperature. These polymers find an extensive application in the health care industry. They are used for making sterilization equipments and is an important part of the chemical industry. High temperature polymers also find application in the manufacturing of heat and shock-resistant objects.

Physical properties of polymers

The physical properties of a polymer such as its strength and flexibility depend on:

• chain length - in general, the longer the chains the stronger the polymer;
• side groups - polar side groups (including those that lead to hydrogen bonding) give stronger attraction between polymer chains, making the polymer stronger;
• branching - straight, unbranched chains can pack together more closely than highly branched chains, giving polymers that have higher density, are more crystalline and therefore stronger;
• cross-linking - if polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to melt.

Applications of Polymers:

Agriculture and Agribusiness

• Polymeric materials are used in and on soil to improve aeration, provide mulch, and promote plant growth and health.

  Medicine

• Many biomaterials, especially heart valve replacements and blood vessels, are made of polymers like Dacron, Teflon and polyurethane.

  Consumer Science

• Plastic containers of all shapes and sizes are light weight and economically less expensive than the more traditional containers. Clothing, floor coverings, garbage disposal bags, and packaging are other polymer applications.

  Industry

• Automobile parts, windshields for fighter planes, pipes, tanks, packing materials, insulation, wood substitutes, adhesives, matrix for composites, and elastomers are all polymer applications used in the industrial market.

  Sports

• Playground equipment, various balls, golf clubs, swimming pools, and protective helmets are often produced from polymers.

Plastic recycling is the process of recovering scrap or waste plastic and reprocessing the material into useful products.Plastic recycling includes taking any type of plastic, sorting it into different polymers and then chipping it and then melting it down into pellets. After this stage, it can then be used to make items of any sort such as plastic chairs and tables. Soft Plastics are also recycled such as polyethylene film and bags.

The term “polymer” is often used as a synonym for plastic, but many other types of molecules — biological and inorganic — are also polymeric. While all plastics are polymers, not all polymers are plastic. Polymers are rarely useful in themselves and are most often modified or compounded with additives (including colours) to form useful materials. The compounded product is generally termed a plastic. Most people have little contact with "polymers" because most articles that they come across are actually modified and coloured and therefore are actually plastics. Polymers can be classified in many ways, based on how they are developed and perform. For this discussion of recycling, an understanding of two basic types of polymers is helpful:

• Thermoplastic polymers can be heated and formed, then heated and formed again and again. The shape of the polymer molecules are generally linear or slightly branched. This means that the molecules can flow under pressure when heated above their melting point.

• Thermoset polymers undergo a chemical change when they are heated, creating a three-dimensional network. After they are heated and formed, these molecules cannot be re-heated and re-formed.

Comparing these types, thermoplastics are much easier to adapt to recycling.

When different types of plastics are melted together, they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications. The two most widely manufactured plastics, polypropylene and polyethylene behave this way, which limits their utility for recycling. Recently, the use of block copolymers as "molecular stitches" or "macromolecular welding flux" has been proposed to overcome the difficulties associated with phase separation during recycling.