Solid State Polymerization Volatile Stripping Mass Transfer Limits in rPET
Solid state polymerization stripping rate is dictated by internal Fickian diffusion limits of condensation by-products and volatiles within rPET pellet cores.
Transesterification Equilibrium
Meaning
Chemical balance achieved during the melt blending of different polyesters determines the final copolymer architecture and material properties. This chemical state, known as transesterification equilibrium, governs the exchange of ester groups between polymer chains. The degree of exchange depends on the processing temperature and the presence of residual catalysts.
Without control, the blend can undergo extensive randomized reaction, turning a block copolymer into a random copolymer.
Chemical Reaction
Exchange reactions are catalyzed by metals like antimony or zinc that are left over from the original polymerization. As the reaction approaches transesterification equilibrium, the separate polymer phases merge into a single phase. This transition can be tracked by measuring the glass transition temperature of the blend.
Process Impact
Thermal history during compounding must be kept within a narrow window to prevent unwanted structural changes. If the melt remains at high temperatures too long, it will reach transesterification equilibrium, which can lower the crystallization rate of the polymer. This alters the cooling behavior of the material during moulding.
Material Behavior
Resulting changes in the molecular structure can significantly alter the mechanical properties of the finished part. A controlled approach to transesterification equilibrium allows the formulator to design blends with optimized impact strength and transparency. This control is essential for producing high-performance packaging films.