Thermal Stability and Melt Processing Rules for Polyhydroxyalkanoate Resins
Polyhydroxyalkanoate processing demands strict moisture control below 200 ppm and residence times under three minutes to prevent cis-elimination chain scission.
Ester Pyrolysis
Meaning
Chemical degradation reactions occurring at elevated temperatures cause the breakdown of polyester chains during melt processing. Thermal cleavage during ester pyrolysis leads to a reduction in molecular weight and a decline in the mechanical properties of the molded part. This reaction is particularly prevalent in bio-based polyesters such as polylactic acid and polyhydroxyalkanoates when they are exposed to temperatures above their melting points for extended periods.
The breakdown produces shorter chain fragments and volatile byproducts that can affect the surface quality of the component.
Reaction Mechanism
Thermal degradation of these polyesters proceeds through a cyclic transition that requires no external catalyst or moisture. When the polymer melt reaches a critical temperature, ester pyrolysis occurs through the cleavage of the carbon-oxygen single bond in the ester group, accompanied by the transfer of a hydrogen atom from the beta-carbon. This unimolecular process generates a vinyl-terminated chain end and a carboxylic acid end group.
The acid group then acts as an autocatalyst, accelerating the degradation of the remaining polymer chains and lowering the melt strength.
Processing Consequence
Polymer melt viscosity drops rapidly as the average chain length decreases due to this thermal degradation. This drop causes the molten plastic to run too freely through the runner and gate, often leading to flashing at the parting line of the mold. The generated volatile gases can become trapped in the mold cavity, creating surface voids, splay marks and silver streaks on the finished part.
Mechanical testing of the degraded parts reveals a loss of tensile strength and impact resistance.
Prevention Strategy
Moisture removal and temperature control are the primary methods used to minimize this degradation. Drying the resin to a low dew point before extrusion reduces the risk of hydrolysis, which can run parallel to ester pyrolysis and further weaken the material. Lowering the barrel temperature of the injection molding machine to the minimum required for uniform melting reduces the thermal stress on the polyester chains.