 In this lesson, we will learn about plasticizers and hardeners. You may be familiar with the term plastics to describe a very broad range of polymers all derived from the products obtained from the fractional distillation of crude oil. For example, polyethene is a polymer of ethene monomers. Ethene is one of the products obtained from the cracking of larger hydrocarbons, which are obtained from the fractional distillation process. Similarly, polyvinyl chloride or PVC is a polymer of vinyl chloride monomers. Vinyl chloride is simply the common name for chloroethene. As you may have learned in a previous lesson, PVC can be used to make window and door frames, plastic hinges, and pipes. All of the listed materials are hard and rigid and serve to provide structural support. PVC can also be used to make wiring insulation and waterproof clothing items such as raincoats and rain boots. These are soft and flexible and cannot provide reliable structural support. How is it possible that these materials with such different properties are made from the same polymer? This is made possible by a plasticizer. When you add a plasticizer to PVC, it makes it softer and more flexible. The raincoat, rain boots, and the wiring insulation are made of plasticized PVC. The harder PVC materials are made of unplasticized PVC or UPVC. So what exactly is a plasticizer? A plasticizer is simply a small molecule that can insert itself between polymer chains. What do you think is the effect of these plasticizers on the strength of the intermolecular forces of attraction between the polymers? Here is a hint. Plasticizers make the polymers further away from one another. Please pause the lesson, think about this, and resume when you are ready. As the polymer chains are further away from one another, the strength of the intermolecular forces of attraction decreases. The polymer chains are also able to slide over the plasticizers, so there is fluidity to the structure. This makes the overall structure softer and a lot more flexible. There are some issues concerning the usage of plasticizers in the production of plasticized PVC products. Some plasticizers can leach or leak out of their products. This is an issue because some plasticizers are known carcinogens. A more serious issue here is that some are used to make children's toys and even baby bottles. A hardener has the opposite effect on a polymer. Introducing a hardener or a cross-linking agent makes a polymer stronger and more rigid. We can think of a cross-linking agent as a bridge. It holds the polymer chains in place and does not allow for much movement of the polymers. Let's think about car tires and your bicycle tires. These are made from rubber obtained from tapping para-rubber trees. Rubber is actually a polymer of isoprene monomers. It would not be able to provide the structural support and strength required to function as tires on its own. Sulphur is added to introduce cross-linking between the polymers in an industrial process known as vulcanization. The presence of these sulfur-sulfur bridges greatly increases the strength and the durability of these tires. Due to the presence of sulfur in these tires, burning rubber can actually form sulfur dioxide which could then combine with rain to form sulfuric acid contributing to acid rain. In summary, a plasticizer makes a polymer softer and more flexible whereas a hardener or a cross-linking agent makes a polymer harder and more rigid.