Moisture in polymers: hygroscopic and non-hygroscopic materials.
All materials polymeric materials during the synthesis, transport and storage phase have a tendency to retain moisture,reaching a equilibrium value with the environment, which depends on the type of polymer, humidity and air temperature, the size of the grain and many other factors that will be studied in detail in the next chapter.
Depending on the ability to absorb water molecules in the surrounding environment, plastics can be divided into: igroscopic and non-igroscopic.
In igroscopic polymers, water is absorbed inside the plastic grain and chemically binds with the material itself.
In non-igroscopic polymers, on the other hand, water does not penetrate the material but only settles on the surface.
The process of removing surface moisture in non-igroscopic materials is quick and easy and requires the use of hot air dryers.
In the case of igroscopic polymers, on the other hand, the removal of residual moisture is more difficult and requires the use of dehumidifiers in which the hot air, infused to remove the water contained in polymer granules, is preemptively dehumidified.
Many technical polymers (also called “technopolymers” or “engineering polymers”) are igroscopic and are characterized by a certain percentage of moisture that makes them saturated and a precise rate of absorption.
When a higroscopic polymer is exposed to the atmosphere, water molecules spread within the polymer structure by binding to molecular chains and causing the reduction of intermolecular bonds and increasing the mobility of molecules, acting as a plasticizer.
In general, the igroscopicity of a polymer is linked to the polarity of the chemical structure of the polymer’s macromolecules.
An important feature of water is the polarity of its molecule, with a molecular dipole moment of 1,847 D.
The water molecule forms an angle of 104.5o with the oxygen atom at the top and the two hydrogen atoms at either end.
Since oxygen has a greater electroniveness,the vertex of the molecule hosts a partial negative electrical charge, while the extremities carry a partial positive electrical charge.
A molecule that presents this imbalance of electrical charges is said to be an electric dipole.
In the molecular structure of many higroscopic polymers is present the carbonyl group, which is a functional group consisting of a carbon atom and one oxygen atom bound by a double bond.
The peculiarity of this group is that oxygen is very electronegative and gives a polarity to the bond.
Since oxygen has a greater electroniveness, it hosts a partial negative electrical charge, while carbon remains a partial positive electrical charge.
Polymers containing many carbonyl groups therefore have a negative charge on oxygen that attracts the positive charge present on the hydrogen atom of the water molecule.
The attraction between the positive and the negative charge generates a weak bond called a hydrogen bridge.
The carbonyl group is present in many igroscopic polymers such as polycarbonate (PC), polyeilentereftalate (PET) and polybutilentereftalate (PBT).
Hydrogen bridge bonds are weak compared to the strong bonds in the polymer chain, but they are strong enough to cause water molecules to absorb up to a equilibrium value that is characteristic for each different type of polymer.
In polyamide, nitrogen-bound hydrogen has a weak positive charge, as the nitrogen atom is much more electronegative than the hydrogen atom, and once attracted by the negative oxygen charge of the water molecule forms a hydrogen bridge bond.
In addition, there is also a carbonyl group in polyammides that forms weak bonds with hydrogen in water molecules.
The higroscopicity of polymers, therefore, is linked to the structure of macromolecules and the formation of hydrogen bridge bonds that cause moisture to rise.
In fact polymers that contain the carbonyl group and polymers such as polyammides are igroscopic and absorb moisture through the formation of hydrogen bonds.
Non-polar polymers, on the other hand, such as polyolefins (polypropylene and polyethylene) and polystyrene do not absorb moisture through hydrogen bonds.