Intro to Polymers
The term polymer is commonly used today in the plastics and composites industry, and it is often used to imply the meaning of "plastic" or "resin". In actuality, the term polymer means much more.
A polymer is chemical compound where molecules are bonded together in long repeating chains. These materials, polymers, have unique properties and can be tailored depending on their intended purpose.
Polymers are both man made and are naturally occurring. For example, rubber is a natural polymeric material that is extremely useful and has been used by man for thousands of years. Rubber has excellent elastic properties, and this is a result of the molecular polymer chain created by mother nature. Both man made and natural polymers can exhibit elastic properties, however, polymers can exhibit a wide range of additional useful properties. Depending on the desired use, polymers can be finely tuned to leverage the advantageous property. These properties include:
- Impact resistant
Polymerization is the method of creating a synthetic polymer by combining many small monomer molecules into chain held together by covalent bonds. There are two major forms of polymerization, step growth polymerization and chain growth polymerization. The main difference between the two types of polymerization is that in chain growth polymerization, monomer molecules are added to the chain one at a time. In the case of step growth polymerization, monomer molecules can bond directly with one another.
It goes without saying that the process of polymerization is full of complexity and unique terminology. Both of which we will not go into in-depth in this particular article.
If one were to look at a polymer chain close up, they would see that the visual structure and physical properties of the molecule chain would mimic the actual physical properties of the polymer.
For example, if the polymer chain is comprised of tightly twisted bonds between monomers and are difficult to break. Chances are this polymer will strong and tough. Or, if a polymer chain on a molecular level exhibits stretchy characteristics, chances are this polymer will have flexible properties as well.
Cross Linked Polymers
Most polymers, commonly referred to as plastics or thermoplastics, are not cross linked polymers. Meaning, the bonds between molecules and polymer chains can be broken and re-attached.
If you think about most common plastics, they can be bent into shapes with heat. They can also be recycled. Plastic soda bottles are melted down and can be reused to make everything from carpet to fleece jackets, or made into new water bottles. This is all done simply with the addition of heat.
Cross linked polymers on the other hand cannot re-bond after the cross linked bond between molecules is broken. Cross linked polymers often exhibit desired properties such as higher strength, rigidity, thermal properties and hardness.
In FRP (Fiber Reinforced Polymer) composite products, cross linked polymers are most commonly used, and they are referred to as resin, or thermoset resin. The most common polymers used in composites are polyester, vinyl ester, and epoxy.
However, perhaps the greatest negative attribute to thermoset resins is the inability of the polymer to be reformed, reshaped, or recycled.
Examples Of Polymers
Below is a list of common polymers used today, their nickname, and frequent uses:
- Polypropylene (PP) - Carpet, upholstery
- Polyethylene low density (LDPE) - Grocery bags
- Polyethylene high density (HDPE) - Detergent bottles, toys
- Poly(vinyl chloride) (PVC) - Piping, decking
- Polystyrene (PS) - Toys, foam
- Polytetrafluoroethylene (PTFE, Teflon) - non-stick pans, electrical insulation
- Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas) - Face shields, skylights
- Poly(vinyl acetate) (PVAc) - Paints, adhesives
- Polychloroprene (cis + trans) (Neoprene) - Wetsuits