D01. Classify the types of synthetic resins based on their thermal behavior and discuss their differences in relation to stability in organic solvents.
The two types we are discussing here are the thermoplastic and the thermoset resins. The difference between these two groups is that the thermoplastic resins become plastic when heated, while the thermoset resins become hard. This difference can easily be understood by accepting that thermoplastic resins consist of linear polymer chains that are mainly held together via weak polar bonds. When these resins are heated, the chains are separated, and the bond strength of the intermolecular bond interaction decreases considerable when a certain temperature (Tg) is reached. When this happens, the resin becomes a liquid. Regarding the thermoset resin, this resin starts cross-linking at a certain temperature, and when such a cross-linkage occurs, the intermolecular bond strength increases, which results in a harder resin.
If the above materials are placed in an organic solvent, the solvent will diffuse into the two resins and start separating the main chains. In the thermoplastic resin, there is no restriction regarding how far the chains can be separated, and therefore, the thermoplastic resin may dissolve completely in this solvent. However, in the thermoset resin case, the cross-links will restrict further chain separation during the initial swelling caused by the solvent, and in this case the resin will not dissolve.
The figure above shows an expansion curve as a function of temperature. At a certain temperature, there is a change in expansion. That temperature is called the glass transition temperature (Tg). At this temperature, the molecules start separating easier because the thermal expansion has now reached a critical distance when some of the attractive forces decrease dramatically. At their Tg, linear polymers become fluids and cross-linked polymers become elastomeric materials (rubber-like materials).
When monomer molecules react and form polymer chains, they may either form linear chains or chains that branch and bond together the linear chains. The later structure is cross-linked, which means that the chains cannot be separated even if stored in a medium that is capable of acting as a solvent. The linear polymer on the other hand would be dissolved, because linear polymer chains are held together by weak polar bonds, hydrogen bonds and mechanical entanglement.
The differences between thermoset and thermoplastic resins can be understood by comparing the effect of heat on a linear and a cross-linked polymer. When a linear polymer reaches a certain temperature, the weak intermolecular bonds break and the chains separate easier. When that happens, these chains can also slip easier in relation to each other. Consequently, the material has become thermoplastic.
If, on the other hand, cross-links form at a certain temperature, the cross-links will restrict further chain-slip, and the material can become harder. In this case we are dealing with a thermoset material.
