Low Frequency Rheological Testing for Hydrolyzed Fraction Identification in Resins
Low frequency oscillatory rheology unmask hydrolyzed resin fractions by measuring zero-shear viscosity drop and storage modulus slope collapse near zero frequency.
Condensation Polymers
Meaning
Macromolecules formed through step growth polymerization represent a primary category of engineering plastics used in durable and packaging applications. These condensation polymers arise when bifunctional or trifunctional monomers react, yielding a polymer chain and a small molecular byproduct like water or alcohol. Unlike addition reactions, each step is an independent chemical event.
The reaction continues until the reactive endgroups are depleted or an equilibrium is reached.
Synthetic Pathway
Chemical synthesis determines the distribution of molecular weight in these materials. Catalysts and temperature profiles control the rate of step growth in condensation polymers to achieve the desired degree of polymerization. Inadequate reaction time results in low molecular weight fractions, which reduce the mechanical strength of the moulded component.
Excessive heating can cause thermal degradation or unwanted crosslinking.
Moisture Sensitivity
The presence of moisture during melt processing initiates hydrolytic cleavage of the ester or amide bonds in the chain. Because condensation polymers are formed by reversible reactions, exposure to water at elevated temperatures reverses the polymerization process. This hydrolysis reduces the average molecular weight, causing a drop in melt viscosity and a reduction in the impact resistance of the finished part.
Thorough desiccant drying before processing is necessary to prevent this degradation. In polyamides and polyesters, the critical moisture threshold is often below 0.02 percent by weight, meaning that even atmospheric humidity can ruin the batch if the resin sits unprotected in a hopper. Moulders run moisture analyzers to verify the dryness of the feed before starting the production run.
Process Control
Operating parameters in the injection moulding cylinder must be controlled to prevent polymer chains from breaking. Moulders adjust screw speed and back pressure to minimize mechanical stress on the polymer. When condensation polymers are processed with excessive shear, the resulting thermal spike causes cosmetic defects such as splay or structural weakness.
Regrind usage must be limited to prevent the accumulation of degraded shorter chains in the molded product.