Meaning
Thermal reactions at extreme temperatures cause the chemical breakdown of polymer chains or additives, leading to the formation and release of water molecules. This pyrolytic water release is distinct from the evaporation of absorbed moisture because it involves the destruction of chemical bonds. It is a common phenomenon in certain flame retardants and condensation polymers like nylon.
Dehydration Step
Breaking down the molecular structure often occurs in specific temperature windows where the bonds are no longer stable. In some plastics, this release is a deliberate part of the flame-retardant mechanism, as the water helps to cool the material and dilute flammable gases. However, in standard moulding, this unexpected water can cause severe processing issues.
Processing Defect
Water generated inside the injection barrel turns into steam and creates splay marks or silver streaks on the surface of the part. This steam can also cause hydrolytic degradation, which reduces the molecular weight and the mechanical strength of the plastic. Managing the melt temperature is the only way to prevent this pyrolytic water release during the run.
Analytical Separation
Distinguishing between surface moisture and pyrolytic water requires advanced thermal analysis techniques like thermogravimetric analysis coupled with a mass spectrometer. This allows the lab to see the exact temperature at which the chemical reaction occurs. Understanding this behavior helps in setting the upper limit for the processing window.
