Solid State  Polymerization

Solid State Polymerization (SSP) is a process in which the polymer chain lengths are increased by heat in the absence of oxygen and water, by means of either vacuum or purging with an  inert gas to drive off the by-products of reactions. The reaction is driven by temperature, pressure, and the diffusion of by-products from the interior of the  pellet to the surface. SSP is an important step frequently used after  melt-polymerization for the purpose of enhancing the mechanical and rheological  properties of polymers before injection blow molding or extruding. The SSP  technique is widely applied in industrial manufacture of bottle-grade PET, films, and superior industrial fibers.

  How the Process  Works

• PPLLC receives PET, polyesters or other condensation polymers made by melt polymerization
• PPLLC transfers the pellets to a Rotary Vacuum Dryer (RVD)
• Generally, the vacuum  pump removes air, water and by-products of the SSP; alternatively, nitrogen purging is used
• The temperature/time profile is controlled with a microprocessor
• Initial heating at a temperature above Tg cold crystallizes amorphous pellets to prevent sticking and fusion of pellets
• Heating is continued to dry the ”wet” pellets
• Heating is continued to a  predetermined temperature below the melting temperature of the  polymer
• Solid  state polymerization is done at a high temperature and low vacuum (or alternatively at zero oxygen level with nitrogen purging) to achieve the desired high molecular weight of the polymer
• The SSP progress is followed by  periodically taking samples with custom designed samplers
• The melt viscosity at a specific temperature of that sample is determined within several minutes
• The Intrinsic Viscosity  is obtained from the Correlation Chart/Curve established from melt viscosity  data and Intrinsic Viscosity obtained from solution viscosity at ambient  temperatures for PET homopolymers
• The final product viscosity is as  specified by the customer
• A major benefit of solid state polymerization is obtaining a desired high molecular weight polymer of uniform and consistent Intrinsic Viscosity
• Other major benefits include the reduction of undesirable side reactions and the levels of by-products
• For PET, the SSP by-products are ethylene glycol and acetaldehyde
• When the desired molecular weight is achieved, the product pellets are cooled and transferred to storage tanks
• The  product pellets are classified, packaged and shipped according to customer  requirements

Precursor Pellets, Lower  Molecular Weight, Transparent, Amorphous, “Wet” Feed Stock Polymer Heated in Vacuum or Inert Atmosphere to Opaque, Crystalline Dry Solids Heat at Higher Temperature in Vacuum or Inert Atmosphere to produce pellets that are opaque, crystalline, dry high molecular weight

               Tg = Glass Transition Temperature Tm = Melting Temperature