Meaning
Chemical breakdown occurs when water molecules physically react with the ester linkages in a polymer backbone to split them into smaller fragments. This process is the primary cause of material failure in polyesters and polyurethanes when they are processed without adequate drying. Melt processing intensifies ester hydrolysis because high heat provides the energy needed for moisture to sever the covalent bonds almost instantly.
Chain Reduction
Scission events triggered by water result in a rapid decline in the average length of the molecules. Shorter chains offer less resistance to movement which allows the melt to become extremely fluid and leak through mold seals. Finished parts made from such compromised resin exhibit brittle behavior and frequently fail during mechanical stress testing.
Atmospheric Sensitivity
Storage conditions play a massive role in the prevalence of this problem before the material ever reaches the machine. Open bags of pellets absorb water from the surrounding humidity through passive diffusion. Even if the dryer is running, ester hydrolysis can happen if the residence time is insufficient or the temperature is too low to drive off deep-seated moisture.
Moulders verify moisture content before starting a run to avoid permanent part damage.
Process Consequence
Low molecular weight byproducts generated during the cleavage can outgas and create silvery streaks known as splay on the part surface. These visual errors often mask deeper structural weaknesses located within the wall of the molded component. Reducing the exposure to humidity is the single most effective way to manage these risks.
