Why Plastics are so Unique ?

 

 

Plastics are everywhere in the world. Their use is increasing in almost all aspects. In this post, we see Plastics and some interesting points regarding them.

What are the Plastics?

Plastics are Organic macromolecular compounds obtained from Polymerization & Polycondensation reactions.

In these reactions, molecules having small molecular weight called monomers combine and polymerize to form macromolecular compounds also called Polymers.

Decoding the definition:

Organic‘ word used here refers to the carbons, hydrogens, etc.

Macromolecular compounds mean large compounds made up of smaller monomers joining together. Please see below image of macromolecule made up of smaller monomer subunits

              Macromolecules, Source- Wikimedia Commons

 

Polymerization reactions  Reactions involving the joining of monomers with the help of chemical bonds. Example- ethylene molecules ( C2H) polymerize together to form the Polyethylene, a polymer very well known.

Polycondensation reactions– During the polymerization reactions, some molecules come out of the reaction mechanism, like removal of H2O, NH3 and alcohol molecules during the reaction.

The main difference between both reactions is that polycondensation reactions give out extra molecules in addition to the polymerization.

First Plastic in the World

The first plastics were made from Oil, Coal & Natural Gas. Coal to methanol was the famous intermediate conversion reaction. Then from methanol, plastic resins are made which were used to mold in desired shapes.

Coal → Syngas → Methanol → Plastic resins (Olefins)

Above shown is an easy procedure of obtaining Plastics from Coal. Firstly, Coal is converted to Syngas then to Methanol, and finally to Olefins by MTO reaction (Methanol-to-Olefins)

Syngas is a mixture of 3 gases (CO + CO2 + H2) obtained by Coal gasification, a process in which coal is converted to Syngas.

 

Polyethylene (1933)

In 1933, polyethylene was invented, which is now the most widely used plastic today in the World.

         Polyethylene 

At that time, this polyethylene was used as

  • Squeeze bottles
  • crates for fish
  • film coating on cardboards to contain milk

Some points about Plastics

1. The composition of plastics can be varied as per requirements.

For example, during the processing of food at deep freezing temperatures (-40°C) or storage of that processed food at -20°C, a different specific plastic is used.

Even if we do Retort sterilization of that stored food at -121°C to kill the microbes or to again preheat that sterilized food in my microwave Oven at 100°C, different plastic materials are used.

So, we can guess the wide applications of plastics in eating processed foods.

                     A microwave oven

2. Generally, most of the plastics are Thermoplastics i.e. they can be repeatedly remolded by melting.

For example, the used plastic bottles shown below are melted and used again to make new chairs or other plastic products. Also, by melting and remolding, we can make various containers, films, and better heat sealable materials.

                 Crushed plastic bottles ready for recycling

3. Thermosetting plastics can be molded only once by application of heat and pressure. We can’t remold them because if we do so, the material starts degrading. Example- Urea-formaldehyde

   Melamine crockery, a thermosetting plastic

4. The inertness of Plastics is very helpful for food industries. Plastics do not react with inorganic chemicals like bases, acids, etc.

Israeli milk bag- bag material does not react with the milk

5. Plastics protect the food from spoilage by acting as a barrier between the outside environment and the food. So, microbes can’t enter into the food and their growth is prohibited.

6. Few plastics are permeable to some gases like O2, N2, CO2 and also water vapors. So we have to prior decide which plastic material we have to choose based on our requirements.

 

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